51 lines
1.8 KiB
PHP
51 lines
1.8 KiB
PHP
A foreign key constraint specifies that the values in a column (or a group of columns) must match the values appearing in some row of another table. In other words foreign key constraints maintain the referential integrity between two related tables.
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Say you have the product table with the following definition:
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<code type='php'>
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class Product extends Doctrine_Record
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{
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public function setTableDefinition()
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{
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$this->hasColumn('id', 'integer', null, 'primary');
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$this->hasColumn('name', 'string');
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$this->hasColumn('price', 'decimal', 18);
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}
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}
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</code>
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Let's also assume you have a table storing orders of those products. We want to ensure that the order table only contains orders of products that actually exist. So we define a foreign key constraint in the orders table that references the products table:
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<code type='php'>
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class Order extends Doctrine_Record
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{
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public function setTableDefinition()
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{
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$this->hasColumn('order_id', 'integer', null, 'primary');
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$this->hasColumn('product_id', 'integer');
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$this->hasColumn('quantity', 'integer');
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}
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public function setUp()
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{
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$this->hasOne('Product', 'Order.product_id');
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// foreign key columns should *always* have indexes
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$this->index('product_id', array('fields' => 'product_id'));
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}
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}
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</code>
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When exported the class 'Order' would execute the following sql:
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CREATE TABLE orders (
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order_id integer PRIMARY KEY,
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product_id integer REFERENCES products (id),
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quantity integer,
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INDEX product_id_idx (product_id)
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)
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Now it is impossible to create orders with product_no entries that do not appear in the products table.
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We say that in this situation the orders table is the referencing table and the products table is the referenced table. Similarly, there are referencing and referenced columns.
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