Added foreign key support to schema builder for MySQL, Postgres, and SQL Server.

Signed-off-by: Taylor Otwell <taylorotwell@gmail.com>

laravel/database/schema/grammars/mysql.php

@@ -197,6 +197,39 @@ class MySQL extends Grammar {
 		return 'ALTER TABLE '.$this->wrap($table)." ADD {$type} {$name}({$keys})";
 	}
 
+	/**
+	 * Generate the SQL statement for creating a foreign key.
+	 *
+	 * @param  Table    $table
+	 * @param  Command  $command
+	 * @return string
+	 */
+	public function foreign(Table $table, Fluent $command)
+	{
+		$name = $command->name;
+
+		// We need to wrap both of the table names in quoted identifiers to protect
+		// against any possible keyword collisions, both the table on which the
+		// command is being executed and the referenced table are wrapped.
+		$table = $this->wrap($table);
+
+		$on = $this->wrap($command->on);
+
+		// Next we need to columnize both the command table's columns as well as
+		// the columns referenced by the foreign key. We'll cast the referenced
+		// columns to an array since they aren't by the fluent command.
+		$foreign = $this->columnize($command->columns);
+
+		$referenced = $this->columnize((array) $command->references);
+
+		// Finally we can built the SQL. This should be the same for all database
+		// platforms we support, but we'll just keep it in the grammars since
+		// adding foreign keys using ALTER isn't supported by SQLite.
+		$sql = "ALTER TABLE $table ADD CONSTRAINT $name ";
+
+		die($sql .= "FOREIGN KEY ($foreign) REFERENCES $on ($referenced)");
+	}
+
 	/**
 	 * Generate the SQL statement for a drop table command.
 	 *

laravel/database/schema/grammars/postgres.php

@@ -194,6 +194,39 @@ class Postgres extends Grammar {
 		return $create." INDEX {$command->name} ON ".$this->wrap($table)." ({$columns})";
 	}
 
+	/**
+	 * Generate the SQL statement for creating a foreign key.
+	 *
+	 * @param  Table    $table
+	 * @param  Command  $command
+	 * @return string
+	 */
+	public function foreign(Table $table, Fluent $command)
+	{
+		$name = $command->name;
+
+		// We need to wrap both of the table names in quoted identifiers to protect
+		// against any possible keyword collisions, both the table on which the
+		// command is being executed and the referenced table are wrapped.
+		$table = $this->wrap($table);
+
+		$on = $this->wrap($command->on);
+
+		// Next we need to columnize both the command table's columns as well as
+		// the columns referenced by the foreign key. We'll cast the referenced
+		// columns to an array since they aren't by the fluent command.
+		$foreign = $this->columnize($command->columns);
+
+		$referenced = $this->columnize((array) $command->references);
+
+		// Finally we can built the SQL. This should be the same for all database
+		// platforms we support, but we'll just keep it in the grammars since
+		// adding foreign keys using ALTER isn't supported by SQLite.
+		$sql = "ALTER TABLE $table ADD CONSTRAINT $name ";
+
+		die($sql .= "FOREIGN KEY ($foreign) REFERENCES $on ($referenced)");
+	}
+
 	/**
 	 * Generate the SQL statement for a drop table command.
 	 *

laravel/database/schema/grammars/sqlserver.php

@@ -212,6 +212,39 @@ class SQLServer extends Grammar {
 		return $create." INDEX {$command->name} ON ".$this->wrap($table)." ({$columns})";
 	}
 
+	/**
+	 * Generate the SQL statement for creating a foreign key.
+	 *
+	 * @param  Table    $table
+	 * @param  Command  $command
+	 * @return string
+	 */
+	public function foreign(Table $table, Fluent $command)
+	{
+		$name = $command->name;
+
+		// We need to wrap both of the table names in quoted identifiers to protect
+		// against any possible keyword collisions, both the table on which the
+		// command is being executed and the referenced table are wrapped.
+		$table = $this->wrap($table);
+
+		$on = $this->wrap($command->on);
+
+		// Next we need to columnize both the command table's columns as well as
+		// the columns referenced by the foreign key. We'll cast the referenced
+		// columns to an array since they aren't by the fluent command.
+		$foreign = $this->columnize($command->columns);
+
+		$referenced = $this->columnize((array) $command->references);
+
+		// Finally we can built the SQL. This should be the same for all database
+		// platforms we support, but we'll just keep it in the grammars since
+		// adding foreign keys using ALTER isn't supported by SQLite.
+		$sql = "ALTER TABLE $table ADD CONSTRAINT $name ";
+
+		die($sql .= "FOREIGN KEY ($foreign) REFERENCES $on ($referenced)");
+	}
+
 	/**
 	 * Generate the SQL statement for a drop table command.
 	 *

laravel/database/schema/table.php

@@ -108,6 +108,17 @@ class Table {
 		return $this->key(__FUNCTION__, $columns, $name);
 	}
 
+	/**
+	 * Add a foreign key constraint to the table.
+	 *
+	 * @param  string|array  $columns
+	 * @param  string        $name
+	 */
+	public function foreign($columns, $name = null)
+	{
+		return $this->key(__FUNCTION__, $columns, $name);
+	}
+
 	/**
 	 * Create a command for creating any index.
 	 *