Move DAL iterator interface and add IteratorEncodeJSON

pkg/dal/driver.go

@@ -90,23 +90,6 @@ type (
 
 	TransformationFunction func(*Model, Attribute, expr.TypedValue) (expr.TypedValue, bool, error)
 
-	// Iterator provides an interface for loading data from the underlying store
-	Iterator interface {
-		Next(ctx context.Context) bool
-		More(uint, ValueGetter) error
-		Err() error
-		Scan(ValueSetter) error
-		Close() error
-
-		BackCursor(ValueGetter) (*filter.PagingCursor, error)
-		ForwardCursor(ValueGetter) (*filter.PagingCursor, error)
-
-		// // -1 means unknown
-		// Total() int
-		// Cursor() any
-		// // ... do we need anything else here?
-	}
-
 	// Store provides an interface which CRS uses to interact with the underlying database
 
 	ValueGetter interface {

pkg/dal/iterator.go

@@ -0,0 +1,65 @@
+package dal
+
+import (
+	"context"
+	"encoding/json"
+	"github.com/cortezaproject/corteza-server/pkg/filter"
+	"io"
+)
+
+type (
+	// Iterator provides an interface for loading data from the underlying store
+	Iterator interface {
+		Next(ctx context.Context) bool
+		More(uint, ValueGetter) error
+		Err() error
+		Scan(ValueSetter) error
+		Close() error
+
+		BackCursor(ValueGetter) (*filter.PagingCursor, error)
+		ForwardCursor(ValueGetter) (*filter.PagingCursor, error)
+
+		// // -1 means unknown
+		// Total() int
+		// Cursor() any
+		// // ... do we need anything else here?
+	}
+)
+
+// IteratorEncodeJSON helper function that encodes each item from the iterator as JSON
+// and writes it to th given io.Writer.
+//
+// target initialization function is intentionally used to avoid use of reflection
+func IteratorEncodeJSON(ctx context.Context, w io.Writer, iter Iterator, initTarget func() ValueSetter) (err error) {
+	var (
+		target   ValueSetter
+		firstOut = false
+	)
+
+	for iter.Next(ctx) {
+		if err = iter.Err(); err != nil {
+			return
+		}
+
+		if firstOut {
+			if _, err = w.Write([]byte(`,`)); err != nil {
+				return
+			}
+		}
+
+		firstOut = true
+
+		target = initTarget()
+
+		if err = iter.Scan(target); err != nil {
+			return
+		}
+
+		err = json.NewEncoder(w).Encode(target)
+		if err != nil {
+			return
+		}
+	}
+
+	return
+}