Adding relationships in NSManagedObjectModel to programmatically created NSEntityDescription

Question

When you write a static library which uses CoreData there\'s a big mess including a normal .xdatamodeld file into the project because you simply cannot just link its compile

Answer 1

Relationships are described by NSRelationshipDescription objects. The following code creates two entity descriptions for "MyCustomEntry", "MyCustomElement" with relationships

• entries (MyCustomElement --> MyCustomEntry, to-many),
• element (MyCustomEntry --> MyCustomElement, to-one), inverse of entries.

Both entities have only a string attribute "identifier" (to save some lines of code).

Objective-c:

NSEntityDescription *entry = [[NSEntityDescription alloc] init];
[entry setName:@"MyCustomEntry"];
[entry setManagedObjectClassName:@"MyCustomEntry"];

NSAttributeDescription *entryIdAttribute = [[NSAttributeDescription alloc] init];
entryIdAttribute.name = @"identifier";
entryIdAttribute.attributeType = NSStringAttributeType;

NSEntityDescription *element = [[NSEntityDescription alloc] init];
[element setName:@"MyCustomElement"];
[element setManagedObjectClassName:@"MyCustomElement"];

NSAttributeDescription *elementIdAttribute = [[NSAttributeDescription alloc] init];
elementIdAttribute.name = @"identifier";
elementIdAttribute.attributeType = NSStringAttributeType;

// To-many relationship from "Element" to "Entry":
NSRelationshipDescription *entriesRelation = [[NSRelationshipDescription alloc] init];

// To-one relationship from "Entry" to "Element":
NSRelationshipDescription *elementRelation = [[NSRelationshipDescription alloc] init];

[entriesRelation setName:@"entries"];
[entriesRelation setDestinationEntity:entry];
[entriesRelation setMinCount:0];
[entriesRelation setMaxCount:0]; // max = 0 for to-many relationship
[entriesRelation setDeleteRule:NSCascadeDeleteRule];
[entriesRelation setInverseRelationship:elementRelation];

[elementRelation setName:@"element"];
[elementRelation setDestinationEntity:element];
[elementRelation setMinCount:0];
[elementRelation setMaxCount:1]; // max = 1 for to-one relationship
[elementRelation setDeleteRule:NSNullifyDeleteRule];
[elementRelation setInverseRelationship:entriesRelation];

[entry setProperties:@[entryIdAttribute, elementRelation]];
[element setProperties:@[elementIdAttribute, entriesRelation]];

NSManagedObjectModel *mom = [[NSManagedObjectModel alloc] init];
[mom setEntities:@[entry, element]];

Swift (now updated for Swift 3/4):

let entry = NSEntityDescription ()
entry.name = "MyCustomEntry"
entry.managedObjectClassName = "MyCustomEntry"

let entryIdAttribute = NSAttributeDescription()
entryIdAttribute.name = "identifier";
entryIdAttribute.attributeType = .stringAttributeType;

let element = NSEntityDescription()
element.name = "MyCustomElement"
element.managedObjectClassName = "MyCustomElement"

let elementIdAttribute =  NSAttributeDescription()
elementIdAttribute.name = "identifier"
elementIdAttribute.attributeType = .stringAttributeType

// To-many relationship from "Element" to "Entry":
let entriesRelation = NSRelationshipDescription()

// To-one relationship from "Entry" to "Element":
let elementRelation = NSRelationshipDescription ()

entriesRelation.name = "entries"
entriesRelation.destinationEntity = entry
entriesRelation.minCount = 0
entriesRelation.maxCount = 0  // max = 0 for to-many relationship
entriesRelation.deleteRule = .cascadeDeleteRule
entriesRelation.inverseRelationship = elementRelation

elementRelation.name = "element"
elementRelation.destinationEntity = element
elementRelation.minCount = 0
elementRelation.maxCount = 1 // max = 1 for to-one relationship
elementRelation.deleteRule = .nullifyDeleteRule
elementRelation.inverseRelationship = entriesRelation

entry.properties = [entryIdAttribute, elementRelation]
element.properties = [elementIdAttribute, entriesRelation]

let mom = NSManagedObjectModel()
mom.entities = [entry, element]

I have tested this code and it seems to work, so I hope that it will be useful to you.