Implement conditional generation of foreign keys.

For now, support is restricted to MySQL and PostgreSQL, while all other
DBMS default to the old behavior.

Signed-off-by: Stephan Peijnik <speijnik@anexia-it.com>

common_dialect.go

@@ -95,6 +95,13 @@ func (c commonDialect) HasIndex(scope *Scope, tableName string, indexName string
 	return count > 0
 }
 
+func (c commonDialect) HasForeignKey(scope *Scope, tableName string, fkName string) bool {
+	// Checking if a foreign key constraint exists is DBMS specific. In order to preserve
+	// the previous logic of AddForeignKey, we are always returning false for dialects which do
+	// not implement this check.
+	return false
+}
+
 func (commonDialect) RemoveIndex(scope *Scope, indexName string) {
 	scope.Err(scope.NewDB().Exec(fmt.Sprintf("DROP INDEX %v ON %v", indexName, scope.QuotedTableName())).Error)
 }

dialect.go

@@ -16,6 +16,7 @@ type Dialect interface {
 	HasTable(scope *Scope, tableName string) bool
 	HasColumn(scope *Scope, tableName string, columnName string) bool
 	HasIndex(scope *Scope, tableName string, indexName string) bool
+	HasForeignKey(scope *Scope, tableName string, fkName string) bool
 	RemoveIndex(scope *Scope, indexName string)
 	CurrentDatabase(scope *Scope) string
 }

mysql.go

@@ -56,6 +56,13 @@ func (mysql) SqlTag(value reflect.Value, size int, autoIncrease bool) string {
 	panic(fmt.Sprintf("invalid sql type %s (%s) for mysql", value.Type().Name(), value.Kind().String()))
 }
 
+func (s mysql) HasForeignKey(scope *Scope, tableName string, fkName string) bool {
+	var count int
+	s.RawScanInt(scope, &count, "SELECT count(*) FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE CONSTRAINT_SCHEMA=? AND TABLE_NAME=? AND CONSTRAINT_NAME=? AND CONSTRAINT_TYPE='FOREIGN KEY'",
+		s.CurrentDatabase(scope), tableName, fkName)
+	return count > 0
+}
+
 func (mysql) Quote(key string) string {
 	return fmt.Sprintf("`%s`", key)
 }

postgres.go

@@ -103,6 +103,13 @@ func (s postgres) HasIndex(scope *Scope, tableName string, indexName string) boo
 	return count > 0
 }
 
+func (s postgres) HasForeignKey(scope *Scope, tableName string, fkName string) bool {
+	var count int
+	s.RawScanInt(scope, &count, "SELECT count(con.conname) FROM pg_constraint con WHERE con.conname = ? AND con.contype='f' AND ?::regclass::oid = con.conrelid",
+		fkName, tableName)
+	return count > 0
+}
+
 func (s postgres) CurrentDatabase(scope *Scope) (name string) {
 	s.RawScanString(scope, &name, "SELECT CURRENT_DATABASE()")
 	return