mirror of
https://github.com/silverstripe/silverstripe-framework
synced 2024-10-22 14:05:37 +02:00
114 lines
5.1 KiB
Markdown
114 lines
5.1 KiB
Markdown
# Database Structure
|
|
|
|
SilverStripe is currently hard-coded to use a fix mapping between data-objects and the underlying database structure -
|
|
opting for "convention over configuration". This page details what that database structure is.
|
|
|
|
## Base tables
|
|
|
|
Each direct sub-class of `[api:DataObject]` will have its own table.
|
|
|
|
The following fields are always created.
|
|
|
|
* ID: Primary Key
|
|
* ClassName: An enumeration listing this data-class and all of its subclasses.
|
|
* Created: A date/time field set to the creation date of this record
|
|
* LastEdited: A date/time field set to the date this record was last edited
|
|
|
|
Every object of this class **or any of its subclasses** will have an entry in this table
|
|
|
|
### Extra Fields
|
|
|
|
* Every field listed in the data object's **$db** array will be included in this table.
|
|
* For every relationship listed in the data object's **$has_one** array, there will be an integer field included in the
|
|
table. This will contain the ID of the data-object being linked to. The database field name will be of the form
|
|
"(relationship-name)ID", for example, ParentID.
|
|
|
|
### ID Generation
|
|
|
|
When a new record is created, we don't use the database's built-in auto-numbering system. Instead, we generate a new ID
|
|
by adding 1 to the current maximum ID.
|
|
|
|
## Subclass tables
|
|
|
|
At SilverStripe's heart is an object-relational model. And a component of object-oriented data is **inheritance**.
|
|
Unfortunately, there is no native way of representing inheritance in a relational database. What we do is store the
|
|
data sub-classed objects across **multiple tables**.
|
|
|
|
For example, suppose we have the following set of classes:
|
|
|
|
* Class `[api:SiteTree]` extends `[api:DataObject]`: Title, Content fields
|
|
* Class `[api:Page]` extends `[api:SiteTree]`: Abstract field
|
|
* Class NewsSection extends `[api:SiteTree]`: *No special fields*
|
|
* Class NewsArticle extend `[api:Page]`: ArticleDate field
|
|
|
|
The data for the following classes would be stored across the following tables:
|
|
|
|
* `[api:SiteTree]`
|
|
* ID: Int
|
|
* ClassName: Enum('SiteTree', 'Page', 'NewsArticle')
|
|
* Created: Datetime
|
|
* LastEdited: Datetime
|
|
* Title: Varchar
|
|
* Content: Text
|
|
* `[api:Page]`
|
|
* ID: Int
|
|
* Abstract: Text
|
|
* NewsArticle
|
|
* ID: Int
|
|
* ArticleDate: Date
|
|
|
|
The way it works is this:
|
|
|
|
* "Base classes" are direct sub-classes of `[api:DataObject]`. They are always given a table, whether or not they have
|
|
special fields. This is called the "base table"
|
|
* The base table's ClassName field is set to class of the given record. It's an enumeration of all possible
|
|
sub-classes of the base class (including the base class itself)
|
|
* Each sub-class of the base object will also be given its own table, *as long as it has custom fields*. In the
|
|
example above, NewsSection didn't have its own data and so an extra table would be redundant.
|
|
* In all the tables, ID is the primary key. A matching ID number is used for all parts of a particular record:
|
|
record #2 in Page refers to the same object as record #2 in `[api:SiteTree]`.
|
|
|
|
To retrieve a news article, SilverStripe joins the `[api:SiteTree]`, `[api:Page]` and NewsArticle tables by their ID fields. We use a
|
|
left-join for robustness; if there is no matching record in Page, we can return a record with a blank Article field.
|
|
|
|
## Staging and versioning
|
|
|
|
[todo]
|
|
|
|
## Schema auto-generation
|
|
|
|
SilverStripe has a powerful tool for automatically building database schemas. We've designed it so that you should never have to build them manually.
|
|
|
|
To access it, visit http://localhost/dev/build?flush=1. This script will analyze the existing schema, compare it to what's required by your data classes, and alter the schema as required.
|
|
|
|
Put the ?flush=1 on the end if you've added PHP files, so that the rest of the system will find these new classes.
|
|
|
|
It will perform the following changes:
|
|
|
|
* Create any missing tables
|
|
* Create any missing fields
|
|
* Create any missing indexes
|
|
* Alter the field type of any existing fields
|
|
* Rename any obsolete tables that it previously created to _obsolete_(tablename)
|
|
|
|
It **won't** do any of the following
|
|
|
|
* Deleting tables
|
|
* Deleting fields
|
|
* Rename any tables that it doesn't recognize - so other applications can co-exist in the same database, as long as their table names don't match a SilverStripe data class.
|
|
|
|
|
|
## Related code
|
|
|
|
The information documented in this page is reflected in a few places in the code:
|
|
|
|
* `[api:DataObject]`
|
|
* requireTable() is responsible for specifying the required database schema
|
|
* instance_get() and instance_get_one() are responsible for generating the database queries for selecting data.
|
|
* write() is responsible for generating the database queries for writing data.
|
|
* `[api:Versioned]`
|
|
* augmentWrite() is responsible for altering the normal database writing operation to handle versions.
|
|
* augmentQuery() is responsible for altering the normal data selection queries to support versions.
|
|
* augmentDatabase() is responsible for specifying the altered database schema to support versions.
|
|
* `[api:MySQLDatabase]`: getNextID() is used when creating new objects; it also handles the mechanics of
|
|
updating the database to have the required schema. |