add custom SQL count support (#609)

* add custom SQL count support

* fix tests

doc.go

@@ -51,11 +51,15 @@ There are 8 major ORM methods and many helpful methods to use to operate databas
     // INSERT INTO struct1 () values ()
     // INSERT INTO struct2 () values (),(),()
 
-2. Query one record from database
+2. Query one record or one variable from database
 
     has, err := engine.Get(&user)
     // SELECT * FROM user LIMIT 1
 
+    var id int64
+    has, err := engine.Table("user").Where("name = ?", name).Get(&id)
+    // SELECT id FROM user WHERE name = ? LIMIT 1
+
 3. Query multiple records from database
 
     var sliceOfStructs []Struct
@@ -99,6 +103,9 @@ another is Rows
     counts, err := engine.Count(&user)
     // SELECT count(*) AS total FROM user
 
+    counts, err := engine.SQL("select count(*) FROM user").Count()
+    // select count(*) FROM user
+
 8. Sum records
 
     sumFloat64, err := engine.Sum(&user, "id")

session_sum.go

@@ -8,7 +8,7 @@ import "database/sql"
 
 // Count counts the records. bean's non-empty fields
 // are conditions.
-func (session *Session) Count(bean interface{}) (int64, error) {
+func (session *Session) Count(bean ...interface{}) (int64, error) {
 	defer session.resetStatement()
 	if session.IsAutoClose {
 		defer session.Close()
@@ -17,7 +17,10 @@ func (session *Session) Count(bean interface{}) (int64, error) {
 	var sqlStr string
 	var args []interface{}
 	if session.Statement.RawSQL == "" {
-		sqlStr, args = session.Statement.genCountSQL(bean)
+		if len(bean) == 0 {
+			return 0, ErrTableNotFound
+		}
+		sqlStr, args = session.Statement.genCountSQL(bean[0])
 	} else {
 		sqlStr = session.Statement.RawSQL
 		args = session.Statement.RawParams

session_sum_test.go

@@ -128,3 +128,25 @@ func TestCount(t *testing.T) {
 	assert.NoError(t, err)
 	assert.EqualValues(t, 1, total)
 }
+
+func TestSQLCount(t *testing.T) {
+	assert.NoError(t, prepareEngine())
+
+	type UserinfoCount2 struct {
+		Id         int64
+		Departname string
+	}
+
+	type UserinfoBooks struct {
+		Id     int64
+		Pid    int64
+		IsOpen bool
+	}
+
+	assertSync(t, new(UserinfoCount2), new(UserinfoBooks))
+
+	total, err := testEngine.SQL("SELECT count(id) FROM userinfo_count2").
+		Count()
+	assert.NoError(t, err)
+	assert.EqualValues(t, 0, total)
+}