<?php

namespace SilverStripe\ORM;

use SilverStripe\Core\Injector\Injector;
use SilverStripe\Dev\Debug;
use SilverStripe\ORM\Filters\SearchFilter;
use SilverStripe\ORM\Queries\SQLConditionGroup;
use SilverStripe\View\ViewableData;
use ArrayIterator;
use Exception;
use InvalidArgumentException;
use LogicException;

/**
 * Implements a "lazy loading" DataObjectSet.
 * Uses {@link DataQuery} to do the actual query generation.
 *
 * DataLists are _immutable_ as far as the query they represent is concerned. When you call a method that
 * alters the query, a new DataList instance is returned, rather than modifying the existing instance
 *
 * When you add or remove an element to the list the query remains the same, but because you have modified
 * the underlying data the contents of the list changes. These are some of those methods:
 *
 *   - add
 *   - addMany
 *   - remove
 *   - removeMany
 *   - removeByID
 *   - removeByFilter
 *   - removeAll
 *
 * Subclasses of DataList may add other methods that have the same effect.
 */
class DataList extends ViewableData implements SS_List, Filterable, Sortable, Limitable
{

    /**
     * The DataObject class name that this data list is querying
     *
     * @var string
     */
    protected $dataClass;

    /**
     * The {@link DataQuery} object responsible for getting this DataList's records
     *
     * @var DataQuery
     */
    protected $dataQuery;

    /**
     * The DataModel from which this DataList comes.
     *
     * @var DataModel
     */
    protected $model;

    /**
     * Create a new DataList.
     * No querying is done on construction, but the initial query schema is set up.
     *
     * @param string $dataClass - The DataObject class to query.
     */
    public function __construct($dataClass)
    {
        $this->dataClass = $dataClass;
        $this->dataQuery = new DataQuery($this->dataClass);

        parent::__construct();
    }

    /**
     * Set the DataModel
     *
     * @param DataModel $model
     */
    public function setDataModel(DataModel $model)
    {
        $this->model = $model;
    }

    /**
     * Get the dataClass name for this DataList, ie the DataObject ClassName
     *
     * @return string
     */
    public function dataClass()
    {
        return $this->dataClass;
    }

    /**
     * When cloning this object, clone the dataQuery object as well
     */
    public function __clone()
    {
        $this->dataQuery = clone $this->dataQuery;
    }

    /**
     * Return a copy of the internal {@link DataQuery} object
     *
     * Because the returned value is a copy, modifying it won't affect this list's contents. If
     * you want to alter the data query directly, use the alterDataQuery method
     *
     * @return DataQuery
     */
    public function dataQuery()
    {
        return clone $this->dataQuery;
    }

    /**
     * @var bool - Indicates if we are in an alterDataQueryCall already, so alterDataQuery can be re-entrant
     */
    protected $inAlterDataQueryCall = false;

    /**
     * Return a new DataList instance with the underlying {@link DataQuery} object altered
     *
     * If you want to alter the underlying dataQuery for this list, this wrapper method
     * will ensure that you can do so without mutating the existing List object.
     *
     * It clones this list, calls the passed callback function with the dataQuery of the new
     * list as it's first parameter (and the list as it's second), then returns the list
     *
     * Note that this function is re-entrant - it's safe to call this inside a callback passed to
     * alterDataQuery
     *
     * @param callable $callback
     * @return static
     * @throws Exception
     */
    public function alterDataQuery($callback)
    {
        if ($this->inAlterDataQueryCall) {
            $list = $this;

            $res = call_user_func($callback, $list->dataQuery, $list);
            if ($res) {
                $list->dataQuery = $res;
            }

            return $list;
        } else {
            $list = clone $this;
            $list->inAlterDataQueryCall = true;

            try {
                $res = call_user_func($callback, $list->dataQuery, $list);
                if ($res) {
                    $list->dataQuery = $res;
                }
            } catch (Exception $e) {
                $list->inAlterDataQueryCall = false;
                throw $e;
            }

            $list->inAlterDataQueryCall = false;
            return $list;
        }
    }

    /**
     * Return a new DataList instance with the underlying {@link DataQuery} object changed
     *
     * @param DataQuery $dataQuery
     * @return static
     */
    public function setDataQuery(DataQuery $dataQuery)
    {
        $clone = clone $this;
        $clone->dataQuery = $dataQuery;
        return $clone;
    }

    /**
     * Returns a new DataList instance with the specified query parameter assigned
     *
     * @param string|array $keyOrArray Either the single key to set, or an array of key value pairs to set
     * @param mixed $val If $keyOrArray is not an array, this is the value to set
     * @return static
     */
    public function setDataQueryParam($keyOrArray, $val = null)
    {
        $clone = clone $this;

        if (is_array($keyOrArray)) {
            foreach ($keyOrArray as $key => $val) {
                $clone->dataQuery->setQueryParam($key, $val);
            }
        } else {
            $clone->dataQuery->setQueryParam($keyOrArray, $val);
        }

        return $clone;
    }

    /**
     * Returns the SQL query that will be used to get this DataList's records.  Good for debugging. :-)
     *
     * @param array $parameters Out variable for parameters required for this query
     * @return string The resulting SQL query (may be paramaterised)
     */
    public function sql(&$parameters = array())
    {
        return $this->dataQuery->query()->sql($parameters);
    }

    /**
     * Return a new DataList instance with a WHERE clause added to this list's query.
     *
     * Supports parameterised queries.
     * See SQLSelect::addWhere() for syntax examples, although DataList
     * won't expand multiple method arguments as SQLSelect does.
     *
     * @param string|array|SQLConditionGroup $filter Predicate(s) to set, as escaped SQL statements or
     * paramaterised queries
     * @return static
     */
    public function where($filter)
    {
        return $this->alterDataQuery(function (DataQuery $query) use ($filter) {
            $query->where($filter);
        });
    }

    /**
     * Return a new DataList instance with a WHERE clause added to this list's query.
     * All conditions provided in the filter will be joined with an OR
     *
     * Supports parameterised queries.
     * See SQLSelect::addWhere() for syntax examples, although DataList
     * won't expand multiple method arguments as SQLSelect does.
     *
     * @param string|array|SQLConditionGroup $filter Predicate(s) to set, as escaped SQL statements or
     * paramaterised queries
     * @return static
     */
    public function whereAny($filter)
    {
        return $this->alterDataQuery(function (DataQuery $query) use ($filter) {
            $query->whereAny($filter);
        });
    }


    /**
     * Returns true if this DataList can be sorted by the given field.
     *
     * @param string $fieldName
     * @return boolean
     */
    public function canSortBy($fieldName)
    {
        return $this->dataQuery()->query()->canSortBy($fieldName);
    }

    /**
     * Returns true if this DataList can be filtered by the given field.
     *
     * @param string $fieldName (May be a related field in dot notation like Member.FirstName)
     * @return boolean
     */
    public function canFilterBy($fieldName)
    {
        $model = singleton($this->dataClass);
        $relations = explode(".", $fieldName);
        // First validate the relationships
        $fieldName = array_pop($relations);
        foreach ($relations as $r) {
            $relationClass = $model->getRelationClass($r);
            if (!$relationClass) {
                return false;
            }
            $model = singleton($relationClass);
            if (!$model) {
                return false;
            }
        }
        // Then check field
        if ($model->hasDatabaseField($fieldName)) {
            return true;
        }
        return false;
    }

    /**
     * Return a new DataList instance with the records returned in this query
     * restricted by a limit clause.
     *
     * @param int $limit
     * @param int $offset
     * @return static
     */
    public function limit($limit, $offset = 0)
    {
        return $this->alterDataQuery(function (DataQuery $query) use ($limit, $offset) {
            $query->limit($limit, $offset);
        });
    }

    /**
     * Return a new DataList instance with distinct records or not
     *
     * @param bool $value
     * @return static
     */
    public function distinct($value)
    {
        return $this->alterDataQuery(function (DataQuery $query) use ($value) {
            $query->distinct($value);
        });
    }

    /**
     * Return a new DataList instance as a copy of this data list with the sort
     * order set.
     *
     * @see SS_List::sort()
     * @see SQLSelect::orderby
     * @example $list = $list->sort('Name'); // default ASC sorting
     * @example $list = $list->sort('Name DESC'); // DESC sorting
     * @example $list = $list->sort('Name', 'ASC');
     * @example $list = $list->sort(array('Name'=>'ASC', 'Age'=>'DESC'));
     *
     * @param String|array Escaped SQL statement. If passed as array, all keys and values are assumed to be escaped.
     * @return static
     */
    public function sort()
    {
        $count = func_num_args();

        if ($count == 0) {
            return $this;
        }

        if ($count > 2) {
            throw new InvalidArgumentException('This method takes zero, one or two arguments');
        }

        if ($count == 2) {
            $col = null;
            $dir = null;
            list($col, $dir) = func_get_args();

            // Validate direction
            if (!in_array(strtolower($dir), array('desc','asc'))) {
                user_error('Second argument to sort must be either ASC or DESC');
            }

            $sort = array($col => $dir);
        } else {
            $sort = func_get_arg(0);
        }

        return $this->alterDataQuery(function (DataQuery $query, DataList $list) use ($sort) {

            if (is_string($sort) && $sort) {
                if (stristr($sort, ' asc') || stristr($sort, ' desc')) {
                    $query->sort($sort);
                } else {
                    $list->applyRelation($sort, $column, true);
                    $query->sort($column, 'ASC');
                }
            } elseif (is_array($sort)) {
                // sort(array('Name'=>'desc'));
                $query->sort(null, null); // wipe the sort

                foreach ($sort as $column => $direction) {
                    // Convert column expressions to SQL fragment, while still allowing the passing of raw SQL
                    // fragments.
                    $list->applyRelation($column, $relationColumn, true);
                    $query->sort($relationColumn, $direction, false);
                }
            }
        });
    }

    /**
     * Return a copy of this list which only includes items with these charactaristics
     *
     * @see SS_List::filter()
     *
     * @example $list = $list->filter('Name', 'bob'); // only bob in the list
     * @example $list = $list->filter('Name', array('aziz', 'bob'); // aziz and bob in list
     * @example $list = $list->filter(array('Name'=>'bob', 'Age'=>21)); // bob with the age 21
     * @example $list = $list->filter(array('Name'=>'bob', 'Age'=>array(21, 43))); // bob with the Age 21 or 43
     * @example $list = $list->filter(array('Name'=>array('aziz','bob'), 'Age'=>array(21, 43)));
     *          // aziz with the age 21 or 43 and bob with the Age 21 or 43
     *
     * Note: When filtering on nullable columns, null checks will be automatically added.
     * E.g. ->filter('Field:not', 'value) will generate '... OR "Field" IS NULL', and
     * ->filter('Field:not', null) will generate '"Field" IS NOT NULL'
     *
     * @todo extract the sql from $customQuery into a SQLGenerator class
     *
     * @param string|array Escaped SQL statement. If passed as array, all keys and values will be escaped internally
     * @return $this
     */
    public function filter()
    {
        // Validate and process arguments
        $arguments = func_get_args();
        switch (sizeof($arguments)) {
            case 1:
                $filters = $arguments[0];

                break;
            case 2:
                $filters = array($arguments[0] => $arguments[1]);

                break;
            default:
                throw new InvalidArgumentException('Incorrect number of arguments passed to filter()');
        }

        return $this->addFilter($filters);
    }

    /**
     * Return a new instance of the list with an added filter
     *
     * @param array $filterArray
     * @return $this
     */
    public function addFilter($filterArray)
    {
        $list = $this;

        foreach ($filterArray as $expression => $value) {
            $filter = $this->createSearchFilter($expression, $value);
            $list = $list->alterDataQuery(array($filter, 'apply'));
        }

        return $list;
    }

    /**
     * Return a copy of this list which contains items matching any of these charactaristics.
     *
     * @example // only bob in the list
     *          $list = $list->filterAny('Name', 'bob');
     *          // SQL: WHERE "Name" = 'bob'
     * @example // azis or bob in the list
     *          $list = $list->filterAny('Name', array('aziz', 'bob');
     *          // SQL: WHERE ("Name" IN ('aziz','bob'))
     * @example // bob or anyone aged 21 in the list
     *          $list = $list->filterAny(array('Name'=>'bob, 'Age'=>21));
     *          // SQL: WHERE ("Name" = 'bob' OR "Age" = '21')
     * @example // bob or anyone aged 21 or 43 in the list
     *          $list = $list->filterAny(array('Name'=>'bob, 'Age'=>array(21, 43)));
     *          // SQL: WHERE ("Name" = 'bob' OR ("Age" IN ('21', '43'))
     * @example // all bobs, phils or anyone aged 21 or 43 in the list
     *          $list = $list->filterAny(array('Name'=>array('bob','phil'), 'Age'=>array(21, 43)));
     *          // SQL: WHERE (("Name" IN ('bob', 'phil')) OR ("Age" IN ('21', '43'))
     *
     * @todo extract the sql from this method into a SQLGenerator class
     *
     * @param string|array See {@link filter()}
     * @return static
     */
    public function filterAny()
    {
        $numberFuncArgs = count(func_get_args());
        $whereArguments = array();

        if ($numberFuncArgs == 1 && is_array(func_get_arg(0))) {
            $whereArguments = func_get_arg(0);
        } elseif ($numberFuncArgs == 2) {
            $whereArguments[func_get_arg(0)] = func_get_arg(1);
        } else {
            throw new InvalidArgumentException('Incorrect number of arguments passed to filterAny()');
        }

        return $this->alterDataQuery(function (DataQuery $query) use ($whereArguments) {
            $subquery = $query->disjunctiveGroup();

            foreach ($whereArguments as $field => $value) {
                $filter = $this->createSearchFilter($field, $value);
                $filter->apply($subquery);
            }
        });
    }

    /**
     * Note that, in the current implementation, the filtered list will be an ArrayList, but this may change in a
     * future implementation.
     * @see Filterable::filterByCallback()
     *
     * @example $list = $list->filterByCallback(function($item, $list) { return $item->Age == 9; })
     * @param callable $callback
     * @return ArrayList (this may change in future implementations)
     */
    public function filterByCallback($callback)
    {
        if (!is_callable($callback)) {
            throw new LogicException(sprintf(
                "SS_Filterable::filterByCallback() passed callback must be callable, '%s' given",
                gettype($callback)
            ));
        }
        /** @var ArrayList $output */
        $output = ArrayList::create();
        foreach ($this as $item) {
            if (call_user_func($callback, $item, $this)) {
                $output->push($item);
            }
        }
        return $output;
    }

    /**
     * Given a field or relation name, apply it safely to this datalist.
     *
     * Unlike getRelationName, this is immutable and will fallback to the quoted field
     * name if not a relation.
     *
     * @param string $field Name of field or relation to apply
     * @param string &$columnName Quoted column name
     * @param bool $linearOnly Set to true to restrict to linear relations only. Set this
     * if this relation will be used for sorting, and should not include duplicate rows.
     * @return $this DataList with this relation applied
     */
    public function applyRelation($field, &$columnName = null, $linearOnly = false)
    {
        // If field is invalid, return it without modification
        if (!$this->isValidRelationName($field)) {
            $columnName = $field;
            return $this;
        }

        // Simple fields without relations are mapped directly
        if (strpos($field, '.') === false) {
            $columnName = '"'.$field.'"';
            return $this;
        }

        return $this->alterDataQuery(
            function (DataQuery $query) use ($field, &$columnName, $linearOnly) {
                $relations = explode('.', $field);
                $fieldName = array_pop($relations);

                // Apply
                $relationModelName = $query->applyRelation($relations, $linearOnly);

                // Find the db field the relation belongs to
                $columnName = DataObject::getSchema()->sqlColumnForField($relationModelName, $fieldName);
            }
        );
    }

    /**
     * Check if the given field specification could be interpreted as an unquoted relation name
     *
     * @param string $field
     * @return bool
     */
    protected function isValidRelationName($field)
    {
        return preg_match('/^[A-Z0-9._]+$/i', $field);
    }

    /**
     * Given a filter expression and value construct a {@see SearchFilter} instance
     *
     * @param string $filter E.g. `Name:ExactMatch:not`, `Name:ExactMatch`, `Name:not`, `Name`
     * @param mixed $value Value of the filter
     * @return SearchFilter
     */
    protected function createSearchFilter($filter, $value)
    {
        // Field name is always the first component
        $fieldArgs = explode(':', $filter);
        $fieldName = array_shift($fieldArgs);

        // Inspect type of second argument to determine context
        $secondArg = array_shift($fieldArgs);
        $modifiers = $fieldArgs;
        if (!$secondArg) {
            // Use default filter if none specified. E.g. `->filter(['Name' => $myname])`
            $filterServiceName = 'DataListFilter.default';
        } else {
            // The presence of a second argument is by default ambiguous; We need to query
            // Whether this is a valid modifier on the default filter, or a filter itself.
            /** @var SearchFilter $defaultFilterInstance */
            $defaultFilterInstance = Injector::inst()->get('DataListFilter.default');
            if (in_array(strtolower($secondArg), $defaultFilterInstance->getSupportedModifiers())) {
                // Treat second (and any subsequent) argument as modifiers, using default filter
                $filterServiceName = 'DataListFilter.default';
                array_unshift($modifiers, $secondArg);
            } else {
                // Second argument isn't a valid modifier, so assume is filter identifier
                $filterServiceName = "DataListFilter.{$secondArg}";
            }
        }

        // Build instance
        return Injector::inst()->create($filterServiceName, $fieldName, $value, $modifiers);
    }

    /**
     * Return a copy of this list which does not contain any items with these charactaristics
     *
     * @see SS_List::exclude()
     * @example $list = $list->exclude('Name', 'bob'); // exclude bob from list
     * @example $list = $list->exclude('Name', array('aziz', 'bob'); // exclude aziz and bob from list
     * @example $list = $list->exclude(array('Name'=>'bob, 'Age'=>21)); // exclude bob that has Age 21
     * @example $list = $list->exclude(array('Name'=>'bob, 'Age'=>array(21, 43))); // exclude bob with Age 21 or 43
     * @example $list = $list->exclude(array('Name'=>array('bob','phil'), 'Age'=>array(21, 43)));
     *          // bob age 21 or 43, phil age 21 or 43 would be excluded
     *
     * @todo extract the sql from this method into a SQLGenerator class
     *
     * @param string|array Escaped SQL statement. If passed as array, all keys and values will be escaped internally
     * @return $this
     */
    public function exclude()
    {
        $numberFuncArgs = count(func_get_args());
        $whereArguments = array();

        if ($numberFuncArgs == 1 && is_array(func_get_arg(0))) {
            $whereArguments = func_get_arg(0);
        } elseif ($numberFuncArgs == 2) {
            $whereArguments[func_get_arg(0)] = func_get_arg(1);
        } else {
            throw new InvalidArgumentException('Incorrect number of arguments passed to exclude()');
        }

        return $this->alterDataQuery(function (DataQuery $query) use ($whereArguments) {
            $subquery = $query->disjunctiveGroup();

            foreach ($whereArguments as $field => $value) {
                $filter = $this->createSearchFilter($field, $value);
                $filter->exclude($subquery);
            }
        });
    }

    /**
     * This method returns a copy of this list that does not contain any DataObjects that exists in $list
     *
     * The $list passed needs to contain the same dataclass as $this
     *
     * @param DataList $list
     * @return static
     * @throws InvalidArgumentException
     */
    public function subtract(DataList $list)
    {
        if ($this->dataClass() != $list->dataClass()) {
            throw new InvalidArgumentException('The list passed must have the same dataclass as this class');
        }

        return $this->alterDataQuery(function (DataQuery $query) use ($list) {
            $query->subtract($list->dataQuery());
        });
    }

    /**
     * Return a new DataList instance with an inner join clause added to this list's query.
     *
     * @param string $table Table name (unquoted and as escaped SQL)
     * @param string $onClause Escaped SQL statement, e.g. '"Table1"."ID" = "Table2"."ID"'
     * @param string $alias - if you want this table to be aliased under another name
     * @param int $order A numerical index to control the order that joins are added to the query; lower order values
     * will cause the query to appear first. The default is 20, and joins created automatically by the
     * ORM have a value of 10.
     * @param array $parameters Any additional parameters if the join is a parameterised subquery
     * @return static
     */
    public function innerJoin($table, $onClause, $alias = null, $order = 20, $parameters = array())
    {
        return $this->alterDataQuery(function (DataQuery $query) use ($table, $onClause, $alias, $order, $parameters) {
            $query->innerJoin($table, $onClause, $alias, $order, $parameters);
        });
    }

    /**
     * Return a new DataList instance with a left join clause added to this list's query.
     *
     * @param string $table Table name (unquoted and as escaped SQL)
     * @param string $onClause Escaped SQL statement, e.g. '"Table1"."ID" = "Table2"."ID"'
     * @param string $alias - if you want this table to be aliased under another name
     * @param int $order A numerical index to control the order that joins are added to the query; lower order values
     * will cause the query to appear first. The default is 20, and joins created automatically by the
     * ORM have a value of 10.
     * @param array $parameters Any additional parameters if the join is a parameterised subquery
     * @return static
     */
    public function leftJoin($table, $onClause, $alias = null, $order = 20, $parameters = array())
    {
        return $this->alterDataQuery(function (DataQuery $query) use ($table, $onClause, $alias, $order, $parameters) {
            $query->leftJoin($table, $onClause, $alias, $order, $parameters);
        });
    }

    /**
     * Return an array of the actual items that this DataList contains at this stage.
     * This is when the query is actually executed.
     *
     * @return array
     */
    public function toArray()
    {
        $query = $this->dataQuery->query();
        $rows = $query->execute();
        $results = array();

        foreach ($rows as $row) {
            $results[] = $this->createDataObject($row);
        }

        return $results;
    }

    /**
     * Return this list as an array and every object it as an sub array as well
     *
     * @return array
     */
    public function toNestedArray()
    {
        $result = array();

        foreach ($this as $item) {
            $result[] = $item->toMap();
        }

        return $result;
    }

    /**
     * Walks the list using the specified callback
     *
     * @param callable $callback
     * @return $this
     */
    public function each($callback)
    {
        foreach ($this as $row) {
            $callback($row);
        }

        return $this;
    }

    public function debug()
    {
        $val = "<h2>" . $this->class . "</h2><ul>";

        foreach ($this->toNestedArray() as $item) {
            $val .= "<li style=\"list-style-type: disc; margin-left: 20px\">" . Debug::text($item) . "</li>";
        }
        $val .= "</ul>";
        return $val;
    }

    /**
     * Returns a map of this list
     *
     * @param string $keyField - the 'key' field of the result array
     * @param string $titleField - the value field of the result array
     * @return Map
     */
    public function map($keyField = 'ID', $titleField = 'Title')
    {
        return new Map($this, $keyField, $titleField);
    }

    /**
     * Create a DataObject from the given SQL row
     *
     * @param array $row
     * @return DataObject
     */
    public function createDataObject($row)
    {
        $class = $this->dataClass;

        if (empty($row['ClassName'])) {
            $row['ClassName'] = $class;
        }

        // Failover from RecordClassName to ClassName
        if (empty($row['RecordClassName'])) {
            $row['RecordClassName'] = $row['ClassName'];
        }

        // Instantiate the class mentioned in RecordClassName only if it exists, otherwise default to $this->dataClass
        if (class_exists($row['RecordClassName'])) {
            $class = $row['RecordClassName'];
        }

        $item = Injector::inst()->create($class, $row, false, $this->model, $this->getQueryParams());

        return $item;
    }

    /**
     * Get query parameters for this list.
     * These values will be assigned as query parameters to newly created objects from this list.
     *
     * @return array
     */
    public function getQueryParams()
    {
        return $this->dataQuery()->getQueryParams();
    }

    /**
     * Returns an Iterator for this DataList.
     * This function allows you to use DataLists in foreach loops
     *
     * @return ArrayIterator
     */
    public function getIterator()
    {
        return new ArrayIterator($this->toArray());
    }

    /**
     * Return the number of items in this DataList
     *
     * @return int
     */
    public function count()
    {
        return $this->dataQuery->count();
    }

    /**
     * Return the maximum value of the given field in this DataList
     *
     * @param string $fieldName
     * @return mixed
     */
    public function max($fieldName)
    {
        return $this->dataQuery->max($fieldName);
    }

    /**
     * Return the minimum value of the given field in this DataList
     *
     * @param string $fieldName
     * @return mixed
     */
    public function min($fieldName)
    {
        return $this->dataQuery->min($fieldName);
    }

    /**
     * Return the average value of the given field in this DataList
     *
     * @param string $fieldName
     * @return mixed
     */
    public function avg($fieldName)
    {
        return $this->dataQuery->avg($fieldName);
    }

    /**
     * Return the sum of the values of the given field in this DataList
     *
     * @param string $fieldName
     * @return mixed
     */
    public function sum($fieldName)
    {
        return $this->dataQuery->sum($fieldName);
    }


    /**
     * Returns the first item in this DataList
     *
     * @return DataObject
     */
    public function first()
    {
        foreach ($this->dataQuery->firstRow()->execute() as $row) {
            return $this->createDataObject($row);
        }
        return null;
    }

    /**
     * Returns the last item in this DataList
     *
     *  @return DataObject
     */
    public function last()
    {
        foreach ($this->dataQuery->lastRow()->execute() as $row) {
            return $this->createDataObject($row);
        }
        return null;
    }

    /**
     * Returns true if this DataList has items
     *
     * @return bool
     */
    public function exists()
    {
        return $this->count() > 0;
    }

    /**
     * Find the first DataObject of this DataList where the given key = value
     *
     * @param string $key
     * @param string $value
     * @return DataObject|null
     */
    public function find($key, $value)
    {
        return $this->filter($key, $value)->first();
    }

    /**
     * Restrict the columns to fetch into this DataList
     *
     * @param array $queriedColumns
     * @return static
     */
    public function setQueriedColumns($queriedColumns)
    {
        return $this->alterDataQuery(function (DataQuery $query) use ($queriedColumns) {
            $query->setQueriedColumns($queriedColumns);
        });
    }

    /**
     * Filter this list to only contain the given Primary IDs
     *
     * @param array $ids Array of integers
     * @return $this
     */
    public function byIDs($ids)
    {
        return $this->filter('ID', $ids);
    }

    /**
     * Return the first DataObject with the given ID
     *
     * @param int $id
     * @return DataObject
     */
    public function byID($id)
    {
        return $this->filter('ID', $id)->first();
    }

    /**
     * Returns an array of a single field value for all items in the list.
     *
     * @param string $colName
     * @return array
     */
    public function column($colName = "ID")
    {
        return $this->dataQuery->column($colName);
    }

    // Member altering methods

    /**
     * Sets the ComponentSet to be the given ID list.
     * Records will be added and deleted as appropriate.
     *
     * @param array $idList List of IDs.
     */
    public function setByIDList($idList)
    {
        $has = array();

        // Index current data
        foreach ($this->column() as $id) {
            $has[$id] = true;
        }

        // Keep track of items to delete
        $itemsToDelete = $has;

        // add items in the list
        // $id is the database ID of the record
        if ($idList) {
            foreach ($idList as $id) {
                unset($itemsToDelete[$id]);
                if ($id && !isset($has[$id])) {
                    $this->add($id);
                }
            }
        }

        // Remove any items that haven't been mentioned
        $this->removeMany(array_keys($itemsToDelete));
    }

    /**
     * Returns an array with both the keys and values set to the IDs of the records in this list.
     * Does not respect sort order. Use ->column("ID") to get an ID list with the current sort.
     *
     * @return array
     */
    public function getIDList()
    {
        $ids = $this->column("ID");
        return $ids ? array_combine($ids, $ids) : array();
    }

    /**
     * Returns a HasManyList or ManyMany list representing the querying of a relation across all
     * objects in this data list.  For it to work, the relation must be defined on the data class
     * that you used to create this DataList.
     *
     * Example: Get members from all Groups:
     *
     *     DataList::Create("Group")->relation("Members")
     *
     * @param string $relationName
     * @return HasManyList|ManyManyList
     */
    public function relation($relationName)
    {
        $ids = $this->column('ID');
        return singleton($this->dataClass)->$relationName()->forForeignID($ids);
    }

    public function dbObject($fieldName)
    {
        return singleton($this->dataClass)->dbObject($fieldName);
    }

    /**
     * Add a number of items to the component set.
     *
     * @param array $items Items to add, as either DataObjects or IDs.
     * @return $this
     */
    public function addMany($items)
    {
        foreach ($items as $item) {
            $this->add($item);
        }
        return $this;
    }

    /**
     * Remove the items from this list with the given IDs
     *
     * @param array $idList
     * @return $this
     */
    public function removeMany($idList)
    {
        foreach ($idList as $id) {
            $this->removeByID($id);
        }
        return $this;
    }

    /**
     * Remove every element in this DataList matching the given $filter.
     *
     * @param string $filter - a sql type where filter
     * @return $this
     */
    public function removeByFilter($filter)
    {
        foreach ($this->where($filter) as $item) {
            $this->remove($item);
        }
        return $this;
    }

    /**
     * Remove every element in this DataList.
     *
     * @return $this
     */
    public function removeAll()
    {
        foreach ($this as $item) {
            $this->remove($item);
        }
        return $this;
    }

    /**
     * This method are overloaded by HasManyList and ManyMany list to perform more sophisticated
     * list manipulation
     *
     * @param mixed $item
     */
    public function add($item)
    {
        // Nothing needs to happen by default
        // TO DO: If a filter is given to this data list then
    }

    /**
     * Return a new item to add to this DataList.
     *
     * @todo This doesn't factor in filters.
     * @param array $initialFields
     * @return DataObject
     */
    public function newObject($initialFields = null)
    {
        $class = $this->dataClass;
        return Injector::inst()->create($class, $initialFields, false, $this->model);
    }

    /**
     * Remove this item by deleting it
     *
     * @param DataObject $item
     * @todo Allow for amendment of this behaviour - for example, we can remove an item from
     * an "ActiveItems" DataList by chaning the status to inactive.
     */
    public function remove($item)
    {
        // By default, we remove an item from a DataList by deleting it.
        $this->removeByID($item->ID);
    }

    /**
     * Remove an item from this DataList by ID
     *
     * @param int $itemID The primary ID
     */
    public function removeByID($itemID)
    {
        $item = $this->byID($itemID);

        if ($item) {
            $item->delete();
        }
    }

    /**
     * Reverses a list of items.
     *
     * @return static
     */
    public function reverse()
    {
        return $this->alterDataQuery(function (DataQuery $query) {
            $query->reverseSort();
        });
    }

    /**
     * Returns whether an item with $key exists
     *
     * @param mixed $key
     * @return bool
     */
    public function offsetExists($key)
    {
        return ($this->limit(1, $key)->first() != null);
    }

    /**
     * Returns item stored in list with index $key
     *
     * @param mixed $key
     * @return DataObject
     */
    public function offsetGet($key)
    {
        return $this->limit(1, $key)->first();
    }

    /**
     * Set an item with the key in $key
     *
     * @param mixed $key
     * @param mixed $value
     */
    public function offsetSet($key, $value)
    {
        user_error("Can't alter items in a DataList using array-access", E_USER_ERROR);
    }

    /**
     * Unset an item with the key in $key
     *
     * @param mixed $key
     */
    public function offsetUnset($key)
    {
        user_error("Can't alter items in a DataList using array-access", E_USER_ERROR);
    }
}