feature: add exists method

Author: demoManito

finisher_api.go

@@ -502,6 +502,14 @@ func (db *DB) Count(count *int64) (tx *DB) {
 	return
 }
 
+// Exists checks if there is any record matching the given conditions
+func (db *DB) Exists() (bool, error) {
+	var exist bool
+	return exist, db.Session(&Session{NewDB: true}).
+		Raw("SELECT ? AS exist", Expr("EXISTS(?)", db.Select("1").Limit(1))).
+		Pluck("exist", &exist).Error
+}
+
 func (db *DB) Row() *sql.Row {
 	tx := db.getInstance().Set("rows", false)
 	tx = tx.callbacks.Row().Execute(tx)

tests/query_test.go

@@ -1453,3 +1453,21 @@ func TestQueryScanToArray(t *testing.T) {
 		t.Error("users[1] should be empty")
 	}
 }
+
+func TestExists(t *testing.T) {
+	ok, err := DB.Table("users").Where("name = ?", "jinzhu").Exists()
+	if err != nil {
+		t.Fatalf("Failed to scan, got %v", err)
+	}
+	if !ok {
+		t.Errorf("Should found record")
+	}
+
+	ok, err = DB.Table("users").Where("name = ?", "jinzhu-jinzhu").Exists()
+	if err != nil {
+		t.Fatalf("Failed to scan, got %v", err)
+	}
+	if ok {
+		t.Errorf("Should not found record")
+	}
+}