Add some docs on maps

docs/content/reference/changesets.md

@@ -0,0 +1,67 @@
+---
+linkTitle: Changesets
+title: Using maps as changesets
+description: Falling back to map[string]interface{} to allow for presence checks.
+menu: { main: { parent: 'reference' } }
+---
+
+Occasionally you need to distinguish presence from nil (undefined vs null). In gqlgen we do this using maps:
+
+
+```graphql
+type Query {
+	updateUser(id: ID!, changes: UserChanges!): User
+}
+
+type UserChanges {
+	name: String
+	email: String
+}
+```
+
+Then in config set the backing type to `map[string]interface{}`
+```yaml
+models:
+  UserChanges:
+    model: "map[string]interface{}"
+```
+
+After running go generate you should end up with a resolver that looks like this:
+```go
+func (r *queryResolver) UpdateUser(ctx context.Context, id int, changes map[string]interface{}) (*User, error) {
+	u := fetchFromDb(id)
+	/// apply the changes
+	saveToDb(u)
+	return u, nil
+}
+```
+
+We often use the mapstructure library to directly apply these changesets directly to the object using reflection:
+```go
+
+func ApplyChanges(changes map[string]interface{}, to interface{}) error {
+	dec, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
+		ErrorUnused: true,
+		TagName:     "json",
+		Result:      to,
+		ZeroFields:  true,
+		// This is needed to get mapstructure to call the gqlgen unmarshaler func for custom scalars (eg Date)
+		DecodeHook: func(a reflect.Type, b reflect.Type, v interface{}) (interface{}, error) {
+			if reflect.PtrTo(b).Implements(reflect.TypeOf((*graphql.Unmarshaler)(nil)).Elem()) {
+				resultType := reflect.New(b)
+				result := resultType.MethodByName("UnmarshalGQL").Call([]reflect.Value{reflect.ValueOf(v)})
+				err, _ := result[0].Interface().(error)
+				return resultType.Elem().Interface(), err
+			}
+
+			return v, nil
+		},
+	})
+
+	if err != nil {
+		return err
+	}
+
+	return dec.Decode(changes)
+}
+```