Performance improvements when updating task assignees, do less unnessecary stuff

REST-Tests/lists.http

@@ -130,12 +130,11 @@ GET http://localhost:8080/api/v1/tasks/caldav
 ###
 
 # Update a task
-POST http://localhost:8080/api/v1/tasks/1
+POST http://localhost:8080/api/v1/tasks/3565
 Authorization: Bearer {{auth_token}}
 Content-Type: application/json
 
 {"assignees":[
-  {"id": 1}
 ]}
 
 ###

pkg/models/list_tasks_create_update.go

@@ -147,22 +147,40 @@ func updateDone(oldTask *ListTask, newTask *ListTask) {
 // Create a bunch of task assignees
 func (t *ListTask) updateTaskAssignees(assignees []*User) (err error) {
 
+	// If we don't have any new assignees, delete everything right away. Saves us some hassle.
+	if len(assignees) == 0 && len(t.Assignees) > 0 {
+		_, err = x.Where("task_id = ?", t.ID).
+			Delete(ListTaskAssginee{})
+		return err
+	}
+
+	// If we didn't change anything (from 0 to zero) don't do anything.
+	if len(assignees) == 0 && len(t.Assignees) == 0 {
+		return nil
+	}
+
+	// Make a hashmap of the new assignees for easier comparision
+	newAssignees := make(map[int64]*User, len(assignees))
+	for _, newAssignee := range assignees {
+		newAssignees[newAssignee.ID] = newAssignee
+	}
+
 	// Get old assignees to delete
 	var found bool
 	var assigneesToDelete []int64
+	oldAssignees := make(map[int64]*User, len(t.Assignees))
 	for _, oldAssignee := range t.Assignees {
 		found = false
-		for _, newAssignee := range assignees {
-			if newAssignee.ID == oldAssignee.ID {
-				found = true // If a new assignee is already in the list with old assignees
-				break
-			}
+		if newAssignees[oldAssignee.ID] != nil {
+			found = true // If a new assignee is already in the list with old assignees
 		}
 
 		// Put all assignees which are only on the old list to the trash
 		if !found {
 			assigneesToDelete = append(assigneesToDelete, oldAssignee.ID)
 		}
+
+		oldAssignees[oldAssignee.ID] = oldAssignee
 	}
 
 	// Delete all assignees not passed
@@ -177,20 +195,17 @@ func (t *ListTask) updateTaskAssignees(assignees []*User) (err error) {
 
 	// Get the list to perform later checks
 	list := List{ID: t.ListID}
-	err = list.ReadOne()
+	err = list.GetSimpleByID()
 	if err != nil {
 		return
 	}
 
 	// Loop through our users and add them
-AddNewAssignee:
 	for _, u := range assignees {
 		// Check if the user is already assigned and assign him only if not
-		for _, oldAssignee := range t.Assignees {
-			if oldAssignee.ID == u.ID {
-				// continue outer loop
-				continue AddNewAssignee
-			}
+		if oldAssignees[u.ID] != nil {
+			// continue outer loop
+			continue
 		}
 
 		// Check if the user exists and has access to the list