Add Distinct method (#27)

Add support for `Distinct` method

Author: thedevsaddam

jsonq.go

@@ -380,6 +380,32 @@ func (j *JSONQ) SortBy(order ...string) *JSONQ {
 	return j.sortBy(order[0], asc)
 }
 
+// Distinct builds distinct value using provided attribute/column/property
+func (j *JSONQ) Distinct(property string) *JSONQ {
+	j.prepare()
+
+	m := map[string]bool{}
+	dt := []interface{}{}
+	if aa, ok := j.jsonContent.([]interface{}); ok {
+		for _, a := range aa {
+			if vm, ok := a.(map[string]interface{}); ok {
+				v, err := getNestedValue(vm, property)
+				if err != nil {
+					j.addError(err)
+				} else {
+					if _, exist := m[toString(v)]; !exist {
+						dt = append(dt, vm)
+						m[toString(v)] = true
+					}
+				}
+			}
+		}
+	}
+	// replace the new result with the previous result
+	j.jsonContent = dt
+	return j
+}
+
 // sortBy sorts list of map
 func (j *JSONQ) sortBy(property string, asc bool) *JSONQ {
 	sortResult, ok := j.jsonContent.([]interface{})

jsonq_test.go

@@ -698,6 +698,27 @@ func TestJSONQ_SortBy_expecting_empty_as_provided_node_is_not_list(t *testing.T)
 	assertJSON(t, out, expJSON)
 }
 
+func TestJSONQ_Distinct(t *testing.T) {
+	jq := New().JSONString(jsonStr).
+		From("vendor.items").
+		Distinct("price")
+	expected := `[{"id":1,"name":"MacBook Pro 13 inch retina","price":1350},{"id":2,"name":"MacBook Pro 15 inch retina","price":1700},{"id":3,"name":"Sony VAIO","price":1200},{"id":4,"name":"Fujitsu","price":850},{"id":6,"name":"HP core i7","price":950}]`
+	out := jq.Get()
+	assertJSON(t, out, expected, "Distinct expecting result")
+}
+
+func TestJSONQ_Distinct_expecting_error(t *testing.T) {
+	jq := New().JSONString(jsonStr).
+		From("vendor.items").
+		Distinct("invalid_key")
+	expected := `[]`
+	out := jq.Get()
+	assertJSON(t, out, expected, "Distinct expecting empty result")
+	if len(jq.Errors()) == 0 {
+		t.Error("failed to catch Distinct error")
+	}
+}
+
 func TestJSONQ_Only(t *testing.T) {
 	jq := New().JSONString(jsonStr).
 		From("vendor.items")