Improve QueryChunks signature to be easier to use

The changes made on this commit were designed by
Raí Tamarindo (raitamarindo@gmail.com) on a previous meeting.

contracts.go

@@ -30,14 +30,9 @@ type ChunkParser struct {
 	Params []interface{}
 
 	ChunkSize int
-	Chunk     interface{} // Must be a pointer to a slice of structs
 
-	// The closure that will be called right after
-	// filling the Chunk with ChunkSize records
-	//
-	// Each chunk consecutively parsed will overwrite the
-	// same slice, so don't keep references to it, if you
-	// need some data to be preserved after all chunks are
-	// processed copy the records by value.
-	ForEachChunk func() error
+	// This attribute must be a func(chunk []<Record>) error,
+	// where the actual Record should be a struct
+	// representing the rows you are expecting to receive.
+	ForEachChunk interface{}
 }

kiss_orm.go

@@ -139,28 +139,33 @@ func (c Client) QueryChunks(
 	}
 	defer rows.Close()
 
-	sliceRef, structType, isSliceOfPtrs, err := decodeAsSliceOfStructs(parser.Chunk)
+	fnValue := reflect.ValueOf(parser.ForEachChunk)
+	chunkType, err := parseInputFunc(parser.ForEachChunk)
 	if err != nil {
 		return err
 	}
 
-	slice := sliceRef.Elem()
-	if slice.Len() > parser.ChunkSize {
-		slice = slice.Slice(0, parser.ChunkSize)
+	chunk := reflect.MakeSlice(chunkType, 0, parser.ChunkSize)
+
+	structType, isSliceOfPtrs, err := decodeAsSliceOfStructs(chunkType)
+	if err != nil {
+		return err
 	}
 
 	var idx = 0
 	for rows.Next() {
-		if slice.Len() <= idx {
+		// Allocate new slice elements
+		// only if they are not already allocated:
+		if chunk.Len() <= idx {
 			var elemValue reflect.Value
 			elemValue = reflect.New(structType)
 			if !isSliceOfPtrs {
 				elemValue = elemValue.Elem()
 			}
-			slice = reflect.Append(slice, elemValue)
+			chunk = reflect.Append(chunk, elemValue)
 		}
 
-		err = c.db.ScanRows(rows, slice.Index(idx).Addr().Interface())
+		err = c.db.ScanRows(rows, chunk.Index(idx).Addr().Interface())
 		if err != nil {
 			return err
 		}
@@ -171,8 +176,7 @@ func (c Client) QueryChunks(
 		}
 
 		idx = 0
-		sliceRef.Elem().Set(slice)
-		err = parser.ForEachChunk()
+		err, _ = fnValue.Call([]reflect.Value{chunk})[0].Interface().(error)
 		if err != nil {
 			if err == ErrAbortIteration {
 				return nil
@@ -184,8 +188,9 @@ func (c Client) QueryChunks(
 	// If no rows were found or idx was reset to 0
 	// on the last iteration skip this last call to ForEachChunk:
 	if idx > 0 {
-		sliceRef.Elem().Set(slice.Slice(0, idx))
-		err = parser.ForEachChunk()
+		chunk = chunk.Slice(0, idx)
+
+		err, _ = fnValue.Call([]reflect.Value{chunk})[0].Interface().(error)
 		if err != nil {
 			if err == ErrAbortIteration {
 				return nil
@@ -435,9 +440,18 @@ func FillStructWith(record interface{}, dbRow map[string]interface{}) error {
 // and the second is a slice of maps representing the database rows you want
 // to use to update this struct.
 func FillSliceWith(entities interface{}, dbRows []map[string]interface{}) error {
-	sliceRef, structType, isSliceOfPtrs, err := decodeAsSliceOfStructs(entities)
+	sliceRef := reflect.ValueOf(entities)
+	sliceType := sliceRef.Type()
+	if sliceType.Kind() != reflect.Ptr {
+		return fmt.Errorf(
+			"FillSliceWith: expected input to be a pointer to struct but got %v",
+			sliceType,
+		)
+	}
+
+	structType, isSliceOfPtrs, err := decodeAsSliceOfStructs(sliceType.Elem())
 	if err != nil {
-		return err
+		return fmt.Errorf("FillSliceWith: %s", err.Error())
 	}
 
 	info, found := tagInfoCache[structType]
@@ -468,34 +482,20 @@ func FillSliceWith(entities interface{}, dbRows []map[string]interface{}) error
 	return nil
 }
 
-func decodeAsSliceOfStructs(slice interface{}) (
-	sliceRef reflect.Value,
+func decodeAsSliceOfStructs(slice reflect.Type) (
 	structType reflect.Type,
 	isSliceOfPtrs bool,
 	err error,
 ) {
-	slicePtrValue := reflect.ValueOf(slice)
-	slicePtrType := slicePtrValue.Type()
-
-	if slicePtrType.Kind() != reflect.Ptr {
+	if slice.Kind() != reflect.Slice {
 		err = fmt.Errorf(
-			"FillListWith: expected input to be a pointer to struct but got %T",
+			"expected input kind to be a slice but got %v",
 			slice,
 		)
 		return
 	}
 
-	t := slicePtrType.Elem()
-
-	if t.Kind() != reflect.Slice {
-		err = fmt.Errorf(
-			"FillListWith: expected input kind to be a slice but got %T",
-			slice,
-		)
-		return
-	}
-
-	elemType := t.Elem()
+	elemType := slice.Elem()
 	isPtr := elemType.Kind() == reflect.Ptr
 
 	if isPtr {
@@ -504,11 +504,39 @@ func decodeAsSliceOfStructs(slice interface{}) (
 
 	if elemType.Kind() != reflect.Struct {
 		err = fmt.Errorf(
-			"FillListWith: expected input to be a slice of structs but got %T",
+			"expected input to be a slice of structs but got %v",
 			slice,
 		)
 		return
 	}
 
-	return slicePtrValue, elemType, isPtr, nil
+	return elemType, isPtr, nil
+}
+
+var errType = reflect.TypeOf(new(error)).Elem()
+
+func parseInputFunc(fn interface{}) (reflect.Type, error) {
+	t := reflect.TypeOf(fn)
+
+	if t.Kind() != reflect.Func {
+		return nil, fmt.Errorf("the ForEachChunk callback must be a function")
+	}
+	if t.NumIn() != 1 {
+		return nil, fmt.Errorf("the ForEachChunk callback must have 1 argument")
+	}
+
+	if t.NumOut() != 1 {
+		return nil, fmt.Errorf("the ForEachChunk callback must have a single return value")
+	}
+
+	if t.Out(0) != errType {
+		return nil, fmt.Errorf("the return value of the ForEachChunk callback must be of type error")
+	}
+
+	argsType := t.In(0)
+	if argsType.Kind() != reflect.Slice {
+		return nil, fmt.Errorf("the argument of the ForEachChunk callback must a slice of structs")
+	}
+
+	return argsType, nil
 }

kiss_orm_test.go

@@ -565,14 +565,12 @@ func TestQueryChunks(t *testing.T) {
 
 		var length int
 		var u User
-		var users []User
 		err = c.QueryChunks(ctx, ChunkParser{
 			Query:  `select * from users where name = ?;`,
 			Params: []interface{}{"User1"},
 
 			ChunkSize: 100,
-			Chunk:     &users,
-			ForEachChunk: func() error {
+			ForEachChunk: func(users []User) error {
 				length = len(users)
 				if length > 0 {
 					u = users[0]
@@ -605,20 +603,23 @@ func TestQueryChunks(t *testing.T) {
 		_ = c.Insert(ctx, &User{Name: "User1"})
 		_ = c.Insert(ctx, &User{Name: "User2"})
 
+		var lengths []int
 		var users []User
 		err = c.QueryChunks(ctx, ChunkParser{
 			Query:  `select * from users where name like ? order by name asc;`,
 			Params: []interface{}{"User%"},
 
 			ChunkSize: 2,
-			Chunk:     &users,
-			ForEachChunk: func() error {
+			ForEachChunk: func(buffer []User) error {
+				users = append(users, buffer...)
+				lengths = append(lengths, len(buffer))
 				return nil
 			},
 		})
 
 		assert.Equal(t, nil, err)
-		assert.Equal(t, 2, len(users))
+		assert.Equal(t, 1, len(lengths))
+		assert.Equal(t, 2, lengths[0])
 		assert.NotEqual(t, 0, users[0].ID)
 		assert.Equal(t, "User1", users[0].Name)
 		assert.NotEqual(t, 0, users[1].ID)
@@ -645,14 +646,12 @@ func TestQueryChunks(t *testing.T) {
 
 		var lengths []int
 		var users []User
-		var buffer []User
 		err = c.QueryChunks(ctx, ChunkParser{
 			Query:  `select * from users where name like ? order by name asc;`,
 			Params: []interface{}{"User%"},
 
 			ChunkSize: 1,
-			Chunk:     &buffer,
-			ForEachChunk: func() error {
+			ForEachChunk: func(buffer []User) error {
 				lengths = append(lengths, len(buffer))
 				users = append(users, buffer...)
 				return nil
@@ -689,14 +688,12 @@ func TestQueryChunks(t *testing.T) {
 
 		var lengths []int
 		var users []User
-		var buffer []User
 		err = c.QueryChunks(ctx, ChunkParser{
 			Query:  `select * from users where name like ? order by name asc;`,
 			Params: []interface{}{"User%"},
 
 			ChunkSize: 2,
-			Chunk:     &buffer,
-			ForEachChunk: func() error {
+			ForEachChunk: func(buffer []User) error {
 				lengths = append(lengths, len(buffer))
 				users = append(users, buffer...)
 				return nil
@@ -735,14 +732,12 @@ func TestQueryChunks(t *testing.T) {
 
 		var lengths []int
 		var users []User
-		var buffer []User
 		err = c.QueryChunks(ctx, ChunkParser{
 			Query:  `select * from users where name like ? order by name asc;`,
 			Params: []interface{}{"User%"},
 
 			ChunkSize: 2,
-			Chunk:     &buffer,
-			ForEachChunk: func() error {
+			ForEachChunk: func(buffer []User) error {
 				lengths = append(lengths, len(buffer))
 				users = append(users, buffer...)
 				return ErrAbortIteration
@@ -780,14 +775,12 @@ func TestQueryChunks(t *testing.T) {
 		returnVals := []error{nil, ErrAbortIteration}
 		var lengths []int
 		var users []User
-		var buffer []User
 		err = c.QueryChunks(ctx, ChunkParser{
 			Query:  `select * from users where name like ? order by name asc;`,
 			Params: []interface{}{"User%"},
 
 			ChunkSize: 2,
-			Chunk:     &buffer,
-			ForEachChunk: func() error {
+			ForEachChunk: func(buffer []User) error {
 				lengths = append(lengths, len(buffer))
 				users = append(users, buffer...)