Groot provides a simple way of serializing Core Data object graphs from or into JSON.

It uses annotations in the Core Data model to perform the serialization and provides the following features:

1. Attribute and relationship mapping to JSON key paths.
2. Value transformation using named NSValueTransformer objects.
3. Object graph preservation.
4. Support for entity inheritance

Getting started

Consider the following JSON describing a well-known comic book character:

{
    "id": "1699",
    "name": "Batman",
    "real_name": "Bruce Wayne",
    "powers": [
        {
            "id": "4",
            "name": "Agility"
        },
        {
            "id": "9",
            "name": "Insanely Rich"
        }
    ],
    "publisher": {
        "id": "10",
        "name": "DC Comics"
    }
}

We could translate this into a Core Data model using three entities: Character, Power and Publisher.

Mapping attributes and relationships

Groot relies on the presence of certain key-value pairs in the user info dictionary associated with entities, attributes and relationships to serialize managed objects from or into JSON. These key-value pairs are often referred in the documentation as annotations.

In our example, we should add a JSONKeyPath in the user info dictionary of each attribute and relationship specifying the corresponding key path in the JSON:

• id for the identifier attribute,
• name for the name attribute,
• real_name for the realName attribute,
• powers for the powers relationship,
• publisher for the publisher relationship,
• etc.

Attributes and relationships that don't have a JSONKeyPath entry are not considered for JSON serialization or deserialization.

Value transformers

When we created the model we decided to use Integer 64 for our identifier attributes. The problem is that, for compatibility reasons, the JSON uses strings for id values.

We can add a JSONTransformerName entry to each identifier attribute's user info dictionary specifying the name of a value transformer that converts strings to numbers.

Groot provides a simple way for creating and registering named value transformers:

// Swift

func toString(_ value: Int) -> String? {
    return String(value)
}

func toInt(_ value: String) -> Int? {
    return Int(value)
}

ValueTransformer.setValueTransformer(withName: "StringToInteger", transform: toString, reverseTransform: toInt)

// Objective-C

[NSValueTransformer grt_setValueTransformerWithName:@"StringToInteger" transformBlock:^id(NSString *value) {
    return @([value integerValue]);
} reverseTransformBlock:^id(NSNumber *value) {
    return [value stringValue];
}];

Object graph preservation

To preserve the object graph and avoid duplicating information when serializing managed objects from JSON, Groot needs to know how to uniquely identify your model objects.

In our example, we should add an identityAttributes entry to the Character, Power and Publisher entities user dictionaries with the value identifier.

For more information about annotating your model have a look at Annotations.

Serializing from JSON

Now that we have our Core Data model ready we can start adding some data.

// Swift

let batmanJSON: JSONDictionary = [
    "name": "Batman",
    "id": "1699",
    "powers": [
        [
            "id": "4",
            "name": "Agility"
        ],
        [
            "id": "9",
            "name": "Insanely Rich"
        ]
    ],
    "publisher": [
        "id": "10",
        "name": "DC Comics"
    ]
]

do {
    let batman: Character = try object(fromJSONDictionary: batmanJSON, inContext: context)
} catch let error as NSError {
    // handle error
}

// Objective-C

Character *batman = [GRTJSONSerialization objectWithEntityName:@"Character"
                                            fromJSONDictionary:batmanJSON
                                                     inContext:self.context
                                                         error:&error];

If we want to update the object we just created, Groot can merge the changes for us:

// Swift

let updateJSON: JSONDictionary = [
    "id": "1699",
    "real_name": "Bruce Wayne",
]

do {
    // This will return the previously created managed object
    let batman: Character = try object(fromJSONDictionary: updateJSON, inContext: context)
} catch let error as NSError {
    // handle error
}

Serializing relationships from identifiers

Suppose that our API does not return full objects for the relationships but only the identifiers.

We don't need to change our model to support this situation:

// Swift

let batmanJSON: JSONDictionary = [
    "name": "Batman",
    "real_name": "Bruce Wayne",
    "id": "1699",
    "powers": ["4", "9"],
    "publisher": "10"
]

do {
    let batman: Character = try object(fromJSONDictionary: batmanJSON, inContext: context)
} catch let error as NSError {
    // handle error
}

The above code creates a full Character object and the corresponding relationships pointing to Power and Publisher objects that just have the identifier attribute populated.

We can import powers and publisher from different JSON objects and Groot will merge them nicely:

// Swift

let powersJSON: JSONArray = [
    [
        "id": "4",
        "name": "Agility"
    ],
    [
        "id": "9",
        "name": "Insanely Rich"
    ]
]

let publisherJSON: JSONDictionary = [
    "id": "10",
    "name": "DC Comics"
]

do {
    let _: [Power] = try objects(fromJSONArray: powersJSON, inContext: context)
    let _: Publisher = try object(fromJSONDictionary: publisherJSON, inContext: context)
} catch let error as NSError {
    // handle error
}

Note that serializing relationships from identifiers only works with entities specifying only one attribute as the value of identityAttributes annotation.

For more serialization alternatives check Groot.swift and GRTJSONSerialization.h.

Entity inheritance

Groot supports entity inheritance via the entityMapperName annotation.

If you are using SQLite as your persistent store, Core Data implements entity inheritance by creating one table for the parent entity and all child entities, with a superset of all their attributes. This can obviously have unintended performance consequences if you have a lot of data in the entities, so use this feature wisely.

Serializing to JSON

Groot provides methods to serialize managed objects back to JSON:

// Swift

let result = json(fromObject: batman)

// Objective-C

NSDictionary *JSONDictionary = [GRTJSONSerialization JSONDictionaryFromObject:batman];

For more serialization alternatives check Groot.swift and GRTJSONSerialization.h.

Contact

Guillermo Gonzalez
@gonzalezreal

License

Groot is available under the MIT license.