Fix dropping foreign key multiple times with test

In some cases the Comparator class returns multiple drops for the same foreign key.
Specifically, in case you have two tables, A & B, with A having a foreign key FK
referencing B, if you drop table B, the resulting diff shows this FK twice,
once on the diff->orphanedForeignKeys array as we're deleting B, and another on
the diff->changedTables array as table A is also being modified. As a result of this you
get the DROP FOREIGN KEY instruction twice in the final SQL.

lib/Doctrine/DBAL/Schema/Comparator.php

@@ -95,6 +95,16 @@ class Comparator
         foreach ($diff->removedTables as $tableName => $table) {
             if (isset($foreignKeysToTable[$tableName])) {
                 $diff->orphanedForeignKeys = array_merge($diff->orphanedForeignKeys, $foreignKeysToTable[$tableName]);
+
+                // deleting duplicated foreign keys present on both on the orphanedForeignKey
+                // and the removedForeignKeys from changedTables
+                foreach ($foreignKeysToTable[$tableName] as $foreignKey) {
+                    if (isset($diff->changedTables[$foreignKey->getLocalTableName()])) {
+                        foreach ($diff->changedTables[$foreignKey->getLocalTableName()]->removedForeignKeys as $key => $removedForeignKey) {
+                            unset($diff->changedTables[$foreignKey->getLocalTableName()]->removedForeignKeys[$key]);
+                        }
+                    }
+                }
             }
         }
 

tests/Doctrine/Tests/DBAL/Schema/ComparatorTest.php

@@ -781,6 +781,32 @@ class ComparatorTest extends \PHPUnit_Framework_TestCase
         $this->assertCount(0, $diff->removedSequences);
     }
 
+
+    /**
+     * You can get multiple drops for a FK when 
+     */
+    public function testAvoidMultipleDropForeignKey()
+    {
+        $oldSchema = new Schema();
+
+        $tableForeign = $oldSchema->createTable('foreign');
+        $tableForeign->addColumn('id', 'integer');
+
+        $table = $oldSchema->createTable('foo');
+        $table->addColumn('fk', 'integer');
+        $table->addForeignKeyConstraint($tableForeign, array('fk'), array('id'));
+
+
+        $newSchema = new Schema();
+        $table = $newSchema->createTable('foo');
+
+        $c = new Comparator();
+        $diff = $c->compare($oldSchema, $newSchema);
+
+        $this->assertCount(0, $diff->changedTables['foo']->removedForeignKeys);
+    }
+
+
     /**
      * @param SchemaDiff $diff
      * @param int $newTableCount