Core Data - Simple JOIN-type Fetch

Question

I am needing to do a "join"-type operation on two tables/models from my Core Data storage, and I'm having a difficult time finding good examples out there for getting something like this to work. It's fairly simple what I need to do. I have two models/tables: locations and location_categories, where locations contains a location/establishment's details (along with a unique ID), and the location_categories table contains category_id's with associated location_id's. I need to select all the locations where, given a category_id, it joins the two tables/models. If it was straight SQL, it would look like this (and this query does work when I run it against the actual SQLite DB): ``` // pretend we passed in a catID of 29: SELECT * FROM zlocation where zlocid in (select zlocid from zloccategory where zcatid = 29) ``` So as you can see, it's pretty straight-forward. Is there a way to do this without having to do two fetches and for-loops? My current objective-c code to do this is below, and i…

Accepted Answer

The best way would be to use relationships.

This way when you fetch a `Location` you can also access the `LocationCategory` like `myLocation.locationCategory`

You would need to set up your relationsship like so:

```
(Entity)LocationCategory.(relationship)locations <->> (Entity)Location.(relationship)locationCategory
```

It's a to-many relationship. Don't forget the inverse either.

Once that's set up you just need to connect relationships up where ever you create the entity instances. Like:

```
 Location *myLocation = ...
 LocationCategory *myLocationCategory = ...
 myLocation.locationCategoy = myLocationCategory;
```

Also make sure you are using the same MOC to fetch the objects. I strongly recommend MagicalRecord to easily manage creating objects and fetching in the specified moc.

Core Data - Simple JOIN-type Fetch

Solution

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