core data in a static library for the iPhone

Sascha's answer got me on the right track. Merging a compiled .mom file from a static library into the .mom file from a host project was relatively simple. Here's a trivial example:

1. Create a new XCode Static Library project called MyStaticLibrary

2. Create an .xcdatamodel file in MyStaticLibrary called MyStaticLibraryModels.xcdatamodel, add some Entitys, then generate the headers and implementations. When you build the MyStaticLibrary target, you'll generate a libMyStaticLibrary.a binary file, but it won't include the compiled .mom file. For that we have to create a bundle.

3. Create a new build target of type Loadable Bundle, found under MacOS X > Cocoa, let's call the new Target MyStaticLibraryModels.

4. Drag MyStaticLibraryModels.xcdatamodel into the Compile Sources build phase of the MyStaticLibraryModels Target. When you build the MyStaticLibraryModels Target, you will generate a file called MyStaticLibraryModels.bundle and it will contain the compiled NSManagedObjectModel file, MyStaticLibraryModels.mom.

5. After building both the MyStaticLibrary and MyStaticLibraryModels Targets, drag libMyStaticLibrary.a (along with any associated Model header files) and MyStaticLibraryModels.bundle into your host project, MyAwesomeApp.

6. MyAwesomeApp uses CoreData, has it's own .xcdatamodel file which will get compiled into a .mom file during its own build process. We want to merge this .mom file with the one we imported in MyStaticLibraryModels.bundle. Somewhere in the MyAwesomeApp project, there is a method that returns MyAwesomeApps NSManagedObjectModel. The Apple generated template for this method looks like this:

...

- (NSManagedObjectModel *)managedObjectModel {
  if (managedObjectModel_ != nil) {
    return managedObjectModel_;
  }
  NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"MyAwesomeApp" withExtension:@"momd"];
  managedObjectModel_ = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];    
  return managedObjectModel_;
}

We will alter this to merge and return BOTH of our NSManagedObjectModels, MyAwesomApps and MyStaticLibraryModels, as a single, combined NSManagedObjectModel like so:

- (NSManagedObjectModel *)managedObjectModel {
  if (managedObjectModel_ != nil) {
    return managedObjectModel_;
  }

  NSMutableArray *allManagedObjectModels = [[NSMutableArray alloc] init];

  NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"MyAwesomeApp" withExtension:@"momd"];
  NSManagedObjectModel *projectManagedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
  [allManagedObjectModels addObject:projectManagedObjectModel];
  [projectManagedObjectModel release];

  NSString *staticLibraryBundlePath = [[NSBundle mainBundle] pathForResource:@"MyStaticLibraryModels" ofType:@"bundle"];
  NSURL *staticLibraryMOMURL = [[NSBundle bundleWithPath:staticLibraryBundlePath] URLForResource:@"MyStaticLibraryModels" withExtension:@"mom"];
  NSManagedObjectModel *staticLibraryMOM = [[NSManagedObjectModel alloc] initWithContentsOfURL:staticLibraryMOMURL];
  [allManagedObjectModels addObject:staticLibraryMOM];
  [staticLibraryMOM release];

  managedObjectModel_ = [NSManagedObjectModel modelByMergingModels:allManagedObjectModels];
  [allManagedObjectModels release];

  return managedObjectModel_;
}

This will return the merged NSManagedObjectModel with the Entitys from both MyAwesomeApp and MyStaticLibrary.

Note that instead of using xcdatamodel/mom file you can also create your model in code (especially if you have a simple model) and this way you won't need to create an additional bundle for resources. Here is a simple example with one table that contains two attributes:

- (NSManagedObjectModel *)coreDataModel
{
    NSManagedObjectModel *model = [NSManagedObjectModel new];

    NSEntityDescription *eventEntity = [NSEntityDescription new];
    eventEntity.name = @"EventEntity";
    eventEntity.managedObjectClassName = @"EventEntity";

    NSAttributeDescription *dateAttribute = [NSAttributeDescription new];
    dateAttribute.name = @"date";
    dateAttribute.attributeType = NSDateAttributeType;
    dateAttribute.optional = NO;

    NSAttributeDescription *typeAttribute = [NSAttributeDescription new];
    typeAttribute.name = @"type";
    typeAttribute.attributeType = NSStringAttributeType;
    typeAttribute.optional = NO;

    eventEntity.properties = @[dateAttribute, typeAttribute];
    model.entities = @[eventEntity];

    return model;
}

Here is a tutorial about creating model from code: https://www.cocoanetics.com/2012/04/creating-a-coredata-model-in-code/

Also based on this approach I created a small and easy to use library that might fit your needs called LSMiniDB so you can check it also.

Also in my case I had warnings such as "warning: dynamic accessors failed to find @property implementation..." on the console while using properties of NSManagedObject subclasses. I was able to fix that by moving those properties to a class interface/implementation instead of having them in a category in a separate file (currently xcode by default is generating this code splited into separate files ClassName+CoreDataClass and ClassName+CoreDataProperties with a class and a category for each subclass).