add mothod Decr

engine.go

@@ -461,6 +461,13 @@ func (engine *Engine) Incr(column string, arg ...interface{}) *Session {
 	return session.Incr(column, arg...)
 }
 
+// Method Inc provides a update string like "column = column - ?"
+func (engine *Engine) Decr(column string, arg ...interface{}) *Session {
+	session := engine.NewSession()
+	session.IsAutoClose = true
+	return session.Decr(column, arg...)
+}
+
 // Temporarily change the Get, Find, Update's table
 func (engine *Engine) Table(tableNameOrBean interface{}) *Session {
 	session := engine.NewSession()

session.go

@@ -146,6 +146,12 @@ func (session *Session) Incr(column string, arg ...interface{}) *Session {
 	return session
 }
 
+// Method In provides a query string like "count = count - 1"
+func (session *Session) Decr(column string, arg ...interface{}) *Session {
+	session.Statement.Decr(column, arg...)
+	return session
+}
+
 // Method Cols provides some columns to special
 func (session *Session) Cols(columns ...string) *Session {
 	session.Statement.Cols(columns...)
@@ -3043,6 +3049,13 @@ func (session *Session) Update(bean interface{}, condiBean ...interface{}) (int6
 		colNames = append(colNames, session.Engine.Quote(v.colName)+" = "+session.Engine.Quote(v.colName)+" + ?")
 		args = append(args, v.arg)
 	}
+	//for update action to like "column = column - ?"
+	decColumns := session.Statement.getDec()
+	for _, v := range decColumns {
+		colNames = append(colNames, session.Engine.Quote(v.colName)+" = "+session.Engine.Quote(v.colName)+" - ?")
+		args = append(args, v.arg)
+	}
+
 	var condiColNames []string
 	var condiArgs []interface{}
 

statement.go

@@ -20,6 +20,11 @@ type incrParam struct {
 	arg     interface{}
 }
 
+type decrParam struct {
+	colName string
+	arg     interface{}
+}
+
 // statement save all the sql info for executing SQL
 type Statement struct {
 	RefTable      *core.Table
@@ -54,6 +59,7 @@ type Statement struct {
 	mustColumnMap map[string]bool
 	inColumns     map[string]*inParam
 	incrColumns   map[string]incrParam
+	decrColumns   map[string]decrParam
 }
 
 // init
@@ -85,6 +91,7 @@ func (statement *Statement) Init() {
 	statement.checkVersion = true
 	statement.inColumns = make(map[string]*inParam)
 	statement.incrColumns = make(map[string]incrParam)
+	statement.decrColumns = make(map[string]decrParam)
 }
 
 // add the raw sql statement
@@ -676,11 +683,27 @@ func (statement *Statement) Incr(column string, arg ...interface{}) *Statement {
 	return statement
 }
 
+// Generate  "Update ... Set column = column - arg" statment
+func (statement *Statement) Decr(column string, arg ...interface{}) *Statement {
+	k := strings.ToLower(column)
+	if len(arg) > 0 {
+		statement.decrColumns[k] = decrParam{column, arg[0]}
+	} else {
+		statement.decrColumns[k] = decrParam{column, 1}
+	}
+	return statement
+}
+
 // Generate  "Update ... Set column = column + arg" statment
 func (statement *Statement) getInc() map[string]incrParam {
 	return statement.incrColumns
 }
 
+// Generate  "Update ... Set column = column - arg" statment
+func (statement *Statement) getDec() map[string]decrParam {
+	return statement.decrColumns
+}
+
 // Generate "Where column IN (?) " statment
 func (statement *Statement) In(column string, args ...interface{}) *Statement {
 	k := strings.ToLower(column)