Core Data: Sort by Relationship's Attribute

Question

I'm building an open-source clone of iPhone's native Messages app called AcaniChat on GitHub.

I have a Conversation entity and a Message entity with a sentDate attribute. Each Conversation can have many Messages. How do I fetch Conversations sorted by the sentDate of it's oldest Message?

Jejune

I've posted a similar solution here: https://mcmap.net/q/1924846/-core-data-problem-select-group-by-having-max – Whoso

Answer 1

The best way I can think of doing this is by adding an attribute to the Conversation entity called lastMessageSentDate and, every time a Message comes in and gets added to a Conversation, setting that conversation's lastMessageSentDate to that message's sentDate. I'll also probably want to add an attribute called lastMessageText to the Conversation entity as well because I want to show the text of the last message for a conversation on the ConversationsViewController (just like the native iPhone Messages app does), and doing so will save me from having to do another Core Data fetch for the Message.

Actually, I just had an idea! Maybe I can sort Conversations by [email protected], according to Xcode Documentation: Key-Value Coding Programming Guide: Collection Operators.

Jejune

Your first idea might be more efficient since it eliminates the need to compute the maximum date for each Conversation.messages collection every time you fetch Conversations! – Anstice

or just have a lastMessage in a Conversation instead of lastMessageSentDate and lastMessageText :) – Benildas

@MattDiPasquale I tried your idea. It didn’t work. See related question: #12749352 – Mcwilliams