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Doctrine has a very powerful abstraction of database schemas. It offers an object-oriented representation of a database schema with support for all the details of Tables, Sequences, Indexes and Foreign Keys. These Schema instances generate a representation that is equal for all the supported platforms. Internally this functionality is used by the ORM Schema Tool to offer you create, drop and update database schema methods from your Doctrine ORM Metadata model. Up to very specific functionality of your database system this allows you to generate SQL code that makes your Domain model work.

You will be pleased to hear, that Schema representation is completly decoupled from the Doctrine ORM though, that is you can also use it in any other project to implement database migrations or for SQL schema generation for any metadata model that your application has. You can easily generate a Schema, as a simple example shows:

$schema = new \Doctrine\DBAL\Schema\Schema();
$myTable = $schema->createTable("my_table");
$myTable->addColumn("id", "integer", array("unsigned" => true));
$myTable->addColumn("username", "string", array("length" => 32));

$myForeign = $schema->createTable("my_foreign");
$myForeign->addColumn("id", "integer");
$myForeign->addColumn("user_id", "integer");
$myForeign->addForeignKeyConstraint($myTable, array("user_id"), array("id"), array("onUpdate" => "CASCADE"));

$queries = $schema->toSql($myPlatform); // get queries to create this schema.
$dropSchema = $schema->toDropSql($myPlatform); // get queries to safely delete this schema.

Now if you want to compare this schema with another schema, you can use the Comparator class to get instances of SchemaDiff, TableDiff and ColumnDiff, as well as information about other foreign key, sequence and index changes.

$comparator = new \Doctrine\DBAL\Schema\Comparator();
$schemaDiff = $comparator->compare($fromSchema, $toSchema);

$queries = $schemaDiff->toSql($myPlatform); // queries to get from one to another schema.
$saveQueries = $schemaDiff->toSaveSql($myPlatform);

The Save Diff mode is a specific mode that prevents the deletion of tables and sequences that might occour when making a diff of your schema. This is often necessary when your target schema is not complete but only describes a subset of your application.

All methods that generate SQL queries for you make much effort to get the order of generation correct, so that no problems will ever occour with missing links of foreign keys.