diff --git a/dialect.go b/dialect.go
index 897e2570..adaf43af 100644
--- a/dialect.go
+++ b/dialect.go
@@ -25,6 +25,8 @@ type Dialect interface {
 
 	// HasIndex check has index or not
 	HasIndex(tableName string, indexName string) bool
+	// HasForeignKey check has foreign key or not
+	HasForeignKey(tableName string, foreignKeyName string) bool
 	// RemoveIndex remove index
 	RemoveIndex(tableName string, indexName string) error
 	// HasTable check has table or not
diff --git a/dialect_common.go b/dialect_common.go
index 7afc5c14..95553c97 100644
--- a/dialect_common.go
+++ b/dialect_common.go
@@ -95,6 +95,10 @@ func (s commonDialect) RemoveIndex(tableName string, indexName string) error {
 	return err
 }
 
+func (s commonDialect) HasForeignKey(tableName string, foreignKeyName string) bool {
+	return false
+}
+
 func (s commonDialect) HasTable(tableName string) bool {
 	var count int
 	s.db.QueryRow("SELECT count(*) FROM INFORMATION_SCHEMA.TABLES WHERE table_schema = ? AND table_name = ?", s.currentDatabase(), tableName).Scan(&count)
diff --git a/dialect_mssql.go b/dialect_mssql.go
deleted file mode 100644
index 7a59bb30..00000000
--- a/dialect_mssql.go
+++ /dev/null
@@ -1,115 +0,0 @@
-package gorm
-
-import (
-	"fmt"
-	"reflect"
-	"strings"
-	"time"
-)
-
-type mssql struct {
-	commonDialect
-}
-
-func init() {
-	RegisterDialect("mssql", &mssql{})
-}
-
-func (mssql) GetName() string {
-	return "mssql"
-}
-
-func (mssql) DataTypeOf(field *StructField) string {
-	var dataValue, sqlType, size, additionalType = ParseFieldStructForDialect(field)
-
-	if sqlType == "" {
-		switch dataValue.Kind() {
-		case reflect.Bool:
-			sqlType = "bit"
-		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uintptr:
-			if _, ok := field.TagSettings["AUTO_INCREMENT"]; ok || field.IsPrimaryKey {
-				sqlType = "int IDENTITY(1,1)"
-			} else {
-				sqlType = "int"
-			}
-		case reflect.Int64, reflect.Uint64:
-			if _, ok := field.TagSettings["AUTO_INCREMENT"]; ok || field.IsPrimaryKey {
-				sqlType = "bigint IDENTITY(1,1)"
-			} else {
-				sqlType = "bigint"
-			}
-		case reflect.Float32, reflect.Float64:
-			sqlType = "float"
-		case reflect.String:
-			if size > 0 && size < 65532 {
-				sqlType = fmt.Sprintf("nvarchar(%d)", size)
-			} else {
-				sqlType = "text"
-			}
-		case reflect.Struct:
-			if _, ok := dataValue.Interface().(time.Time); ok {
-				sqlType = "datetime2"
-			}
-		default:
-			if _, ok := dataValue.Interface().([]byte); ok {
-				if size > 0 && size < 65532 {
-					sqlType = fmt.Sprintf("varchar(%d)", size)
-				} else {
-					sqlType = "text"
-				}
-			}
-		}
-	}
-
-	if sqlType == "" {
-		panic(fmt.Sprintf("invalid sql type %s (%s) for mssql", dataValue.Type().Name(), dataValue.Kind().String()))
-	}
-
-	if strings.TrimSpace(additionalType) == "" {
-		return sqlType
-	}
-	return fmt.Sprintf("%v %v", sqlType, additionalType)
-}
-
-func (s mssql) HasIndex(tableName string, indexName string) bool {
-	var count int
-	s.db.QueryRow("SELECT count(*) FROM sys.indexes WHERE name=? AND object_id=OBJECT_ID(?)", indexName, tableName).Scan(&count)
-	return count > 0
-}
-
-func (s mssql) RemoveIndex(tableName string, indexName string) error {
-	_, err := s.db.Exec(fmt.Sprintf("DROP INDEX %v ON %v", indexName, s.Quote(tableName)))
-	return err
-}
-
-func (s mssql) HasTable(tableName string) bool {
-	var count int
-	s.db.QueryRow("SELECT count(*) FROM INFORMATION_SCHEMA.tables WHERE table_name = ? AND table_catalog = ?", tableName, s.currentDatabase()).Scan(&count)
-	return count > 0
-}
-
-func (s mssql) HasColumn(tableName string, columnName string) bool {
-	var count int
-	s.db.QueryRow("SELECT count(*) FROM information_schema.columns WHERE table_catalog = ? AND table_name = ? AND column_name = ?", s.currentDatabase(), tableName, columnName).Scan(&count)
-	return count > 0
-}
-
-func (s mssql) currentDatabase() (name string) {
-	s.db.QueryRow("SELECT DB_NAME() AS [Current Database]").Scan(&name)
-	return
-}
-
-func (mssql) LimitAndOffsetSQL(limit, offset int) (sql string) {
-	if limit > 0 || offset > 0 {
-		if offset < 0 {
-			offset = 0
-		}
-
-		sql += fmt.Sprintf(" OFFSET %d ROWS", offset)
-
-		if limit >= 0 {
-			sql += fmt.Sprintf(" FETCH NEXT %d ROWS ONLY", limit)
-		}
-	}
-	return
-}
diff --git a/dialect_mysql.go b/dialect_mysql.go
index d98c33a3..a6946d5c 100644
--- a/dialect_mysql.go
+++ b/dialect_mysql.go
@@ -97,6 +97,12 @@ func (s mysql) RemoveIndex(tableName string, indexName string) error {
 	return err
 }
 
+func (s mysql) HasForeignKey(tableName string, foreignKeyName string) bool {
+	var count int
+	s.db.QueryRow("SELECT count(*) FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE CONSTRAINT_SCHEMA=? AND TABLE_NAME=? AND CONSTRAINT_NAME=? AND CONSTRAINT_TYPE='FOREIGN KEY'", s.currentDatabase(), foreignKeyName).Scan(&count)
+	return count > 0
+}
+
 func (s mysql) currentDatabase() (name string) {
 	s.db.QueryRow("SELECT DATABASE()").Scan(&name)
 	return
diff --git a/dialect_postgres.go b/dialect_postgres.go
index 96627d92..baf76659 100644
--- a/dialect_postgres.go
+++ b/dialect_postgres.go
@@ -83,6 +83,12 @@ func (s postgres) HasIndex(tableName string, indexName string) bool {
 	return count > 0
 }
 
+func (s postgres) HasForeignKey(tableName string, foreignKeyName string) bool {
+	var count int
+	s.db.QueryRow("SELECT count(con.conname) FROM pg_constraint con WHERE $1::regclass::oid = con.conrelid AND con.conname = $2 AND con.contype='f'", s.currentDatabase(), foreignKeyName).Scan(&count)
+	return count > 0
+}
+
 func (s postgres) HasTable(tableName string) bool {
 	var count int
 	s.db.QueryRow("SELECT count(*) FROM INFORMATION_SCHEMA.tables WHERE table_name = $1 AND table_type = 'BASE TABLE'", tableName).Scan(&count)
diff --git a/dialects/mssql/mssql.go b/dialects/mssql/mssql.go
index d628ef7c..657bada9 100644
--- a/dialects/mssql/mssql.go
+++ b/dialects/mssql/mssql.go
@@ -1,7 +1,11 @@
 package mssql
 
 import (
+	"database/sql"
 	"fmt"
+	"reflect"
+	"strings"
+	"time"
 
 	_ "github.com/denisenkom/go-mssqldb"
 	"github.com/jinzhu/gorm"
@@ -15,4 +19,132 @@ func setIdentityInsert(scope *gorm.Scope) {
 
 func init() {
 	gorm.DefaultCallback.Create().After("gorm:begin_transaction").Register("mssql:set_identity_insert", setIdentityInsert)
+	gorm.RegisterDialect("mssql", &mssql{})
+}
+
+type mssql struct {
+	db *sql.DB
+}
+
+func (mssql) GetName() string {
+	return "mssql"
+}
+
+func (s *mssql) SetDB(db *sql.DB) {
+	s.db = db
+}
+
+func (mssql) BindVar(i int) string {
+	return "$$" // ?
+}
+
+func (mssql) Quote(key string) string {
+	return fmt.Sprintf(`"%s"`, key)
+}
+
+func (mssql) DataTypeOf(field *gorm.StructField) string {
+	var dataValue, sqlType, size, additionalType = gorm.ParseFieldStructForDialect(field)
+
+	if sqlType == "" {
+		switch dataValue.Kind() {
+		case reflect.Bool:
+			sqlType = "bit"
+		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uintptr:
+			if _, ok := field.TagSettings["AUTO_INCREMENT"]; ok || field.IsPrimaryKey {
+				sqlType = "int IDENTITY(1,1)"
+			} else {
+				sqlType = "int"
+			}
+		case reflect.Int64, reflect.Uint64:
+			if _, ok := field.TagSettings["AUTO_INCREMENT"]; ok || field.IsPrimaryKey {
+				sqlType = "bigint IDENTITY(1,1)"
+			} else {
+				sqlType = "bigint"
+			}
+		case reflect.Float32, reflect.Float64:
+			sqlType = "float"
+		case reflect.String:
+			if size > 0 && size < 65532 {
+				sqlType = fmt.Sprintf("nvarchar(%d)", size)
+			} else {
+				sqlType = "text"
+			}
+		case reflect.Struct:
+			if _, ok := dataValue.Interface().(time.Time); ok {
+				sqlType = "datetime2"
+			}
+		default:
+			if _, ok := dataValue.Interface().([]byte); ok {
+				if size > 0 && size < 65532 {
+					sqlType = fmt.Sprintf("varchar(%d)", size)
+				} else {
+					sqlType = "text"
+				}
+			}
+		}
+	}
+
+	if sqlType == "" {
+		panic(fmt.Sprintf("invalid sql type %s (%s) for mssql", dataValue.Type().Name(), dataValue.Kind().String()))
+	}
+
+	if strings.TrimSpace(additionalType) == "" {
+		return sqlType
+	}
+	return fmt.Sprintf("%v %v", sqlType, additionalType)
+}
+
+func (s mssql) HasIndex(tableName string, indexName string) bool {
+	var count int
+	s.db.QueryRow("SELECT count(*) FROM sys.indexes WHERE name=? AND object_id=OBJECT_ID(?)", indexName, tableName).Scan(&count)
+	return count > 0
+}
+
+func (s mssql) RemoveIndex(tableName string, indexName string) error {
+	_, err := s.db.Exec(fmt.Sprintf("DROP INDEX %v ON %v", indexName, s.Quote(tableName)))
+	return err
+}
+
+func (s mssql) HasForeignKey(tableName string, foreignKeyName string) bool {
+	return false
+}
+
+func (s mssql) HasTable(tableName string) bool {
+	var count int
+	s.db.QueryRow("SELECT count(*) FROM INFORMATION_SCHEMA.tables WHERE table_name = ? AND table_catalog = ?", tableName, s.currentDatabase()).Scan(&count)
+	return count > 0
+}
+
+func (s mssql) HasColumn(tableName string, columnName string) bool {
+	var count int
+	s.db.QueryRow("SELECT count(*) FROM information_schema.columns WHERE table_catalog = ? AND table_name = ? AND column_name = ?", s.currentDatabase(), tableName, columnName).Scan(&count)
+	return count > 0
+}
+
+func (s mssql) currentDatabase() (name string) {
+	s.db.QueryRow("SELECT DB_NAME() AS [Current Database]").Scan(&name)
+	return
+}
+
+func (mssql) LimitAndOffsetSQL(limit, offset int) (sql string) {
+	if limit > 0 || offset > 0 {
+		if offset < 0 {
+			offset = 0
+		}
+
+		sql += fmt.Sprintf(" OFFSET %d ROWS", offset)
+
+		if limit >= 0 {
+			sql += fmt.Sprintf(" FETCH NEXT %d ROWS ONLY", limit)
+		}
+	}
+	return
+}
+
+func (mssql) SelectFromDummyTable() string {
+	return ""
+}
+
+func (mssql) LastInsertIdReturningSuffix(tableName, columnName string) string {
+	return ""
 }
diff --git a/scope_private.go b/scope_private.go
index d0597c11..b2f504d1 100644
--- a/scope_private.go
+++ b/scope_private.go
@@ -616,6 +616,10 @@ func (scope *Scope) addIndex(unique bool, indexName string, column ...string) {
 func (scope *Scope) addForeignKey(field string, dest string, onDelete string, onUpdate string) {
 	var keyName = fmt.Sprintf("%s_%s_%s_foreign", scope.TableName(), field, dest)
 	keyName = regexp.MustCompile("(_*[^a-zA-Z]+_*|_+)").ReplaceAllString(keyName, "_")
+
+	if scope.Dialect().HasForeignKey(scope.TableName(), keyName) {
+		return
+	}
 	var query = `ALTER TABLE %s ADD CONSTRAINT %s FOREIGN KEY (%s) REFERENCES %s ON DELETE %s ON UPDATE %s;`
 	scope.Raw(fmt.Sprintf(query, scope.QuotedTableName(), scope.quoteIfPossible(keyName), scope.quoteIfPossible(field), dest, onDelete, onUpdate)).Exec()
 }