NSManagedObjectModel Class Reference
| Inherits from | |
| Conforms to | |
| Framework | /System/Library/Frameworks/CoreData.framework |
| Availability | Available in iOS 3.0 and later. |
| Declared in | NSManagedObjectModel.h |
| Companion guides | |
Overview
An NSManagedObjectModel object describes a schema—a collection of entities (data models) that you use in your application.
The model contains one or more NSEntityDescription objects representing the entities in the schema. Each NSEntityDescription object has property description objects (instances of subclasses of NSPropertyDescription) that represent the properties (or fields) of the entity in the schema. The Core Data framework uses this description in several ways:
Constraining UI creation in Interface Builder
Validating attribute and relationship values at runtime
Mapping between your managed objects and a database or file-based schema for object persistence.
A managed object model maintains a mapping between each of its entity objects and a corresponding managed object class for use with the persistent storage mechanisms in the Core Data Framework. You can determine the entity for a particular managed object with the entity method.
You typically create managed object models using the data modeling tool in Xcode, but it is possible to build a model programmatically if needed.
Loading a Model File
Managed object model files are typically stored in a project or a framework. To load a model, you provide an URL to the constructor. Note that loading a model doesn’t have the effect of loading all of its entities.
Stored Fetch Requests
It is often the case that in your application you want to get hold of a collection of objects that share features in common. Sometimes you can define those features (property values) in advance; sometimes you need to be able to supply values at runtime. For example, you might want to be able to retrieve all movies owned by Pixar; alternatively you might want to be able to retrieve all movies that earned more than an amount specified by the user at runtime.
Fetch requests are often predefined in a managed object model as templates. They allow you to pre-define named queries and their parameters in the model. Typically they contain variables that need to be substituted at runtime. NSManagedObjectModel provides API to retrieve a stored fetch request by name, and to perform variable substitution—see fetchRequestTemplateForName: and fetchRequestFromTemplateWithName:substitutionVariables:. You can create fetch request templates programmatically, and associate them with a model using setFetchRequestTemplate:forName:; typically, however, you define them using the Xcode design tool.
Configurations
Sometimes a model—particularly one in a framework—may be used in different situations, and you may want to specify different sets of entities to be used in different situations. There might, for example, be certain entities that should only be available if a user has administrative privileges. To support this requirement, a model may have more than one configuration. Each configuration is named, and has an associated set of entities. The sets may overlap. You establish configurations programmatically using setEntities:forConfiguration: or using the Xcode design tool, and retrieve the entities for a given configuration name using entitiesForConfiguration:.
Changing Models
Since a model describes the structure of the data in a persistent store, changing any parts of a model that alters the schema renders it incompatible with (and so unable to open) the stores it previously created. If you change your schema, you therefore need to migrate the data in existing stores to new version (see Core Data Model Versioning and Data Migration Programming Guide). For example, if you add a new entity or a new attribute to an existing entity, you will not be able to open old stores; if you add a validation constraint or set a new default value for an attribute, you will be able to open old stores.
Editing Models Programmatically
Managed object models are editable until they are used by an object graph manager (a managed object context or a persistent store coordinator). This allows you to create or modify them dynamically. However, once a model is being used, it must not be changed. This is enforced at runtime—when the object manager first fetches data using a model, the whole of that model becomes uneditable. Any attempt to mutate a model or any of its sub-objects after that point causes an exception to be thrown. If you need to modify a model that is in use, create a copy, modify the copy, and then discard the objects with the old model.
Fast Enumeration
In OS X v10.5 and later and on iOS, NSManagedObjectModel supports the NSFastEnumeration protocol. You can use this to enumerate over a model’s entities, as illustrated in the following example:
NSManagedObjectModel *aModel = ...; |
for (NSEntityDescription *entity in aModel) { |
// entity is each instance of NSEntityDescription in aModel in turn |
} |
Tasks
Initializing a Model
-
– initWithContentsOfURL: -
+ mergedModelFromBundles: -
+ mergedModelFromBundles:forStoreMetadata: -
+ modelByMergingModels: -
+ modelByMergingModels:forStoreMetadata:
Entities and Configurations
-
– entities -
– entitiesByName -
– setEntities: -
– configurations -
– entitiesForConfiguration: -
– setEntities:forConfiguration:
Getting Fetch Request Templates
-
– fetchRequestTemplatesByName -
– fetchRequestTemplateForName: -
– fetchRequestFromTemplateWithName:substitutionVariables: -
– setFetchRequestTemplate:forName:
Localization
Versioning and Migration
Class Methods
mergedModelFromBundles:
Returns a model created by merging all the models found in given bundles.
Parameters
- bundles
An array of instances of
NSBundleto search. If you specifynil, then the main bundle is searched.
Return Value
A model created by merging all the models found in bundles.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hmergedModelFromBundles:forStoreMetadata:
Returns a merged model from a specified array for the version information in provided metadata.
Parameters
- bundles
An array of bundles.
- metadata
A dictionary containing version information from the metadata for a persistent store.
Return Value
The managed object model used to create the store for the metadata. If a model cannot be created to match the version information specified by metadata, returns nil.
Discussion
This method is a companion to mergedModelFromBundles:.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hmodelByMergingModels:
Creates a single model from an array of existing models.
Parameters
- models
An array of instances of
NSManagedObjectModel.
Return Value
A single model made by combining the models in models.
Discussion
You use this method to combine multiple models (typically from different frameworks) into one.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hmodelByMergingModels:forStoreMetadata:
Returns, for the version information in given metadata, a model merged from a given array of models.
Parameters
- models
An array of instances of
NSManagedObjectModel.- metadata
A dictionary containing version information from the metadata for a persistent store.
Return Value
A merged model from models for the version information in metadata. If a model cannot be created to match the version information in metadata, returns nil.
Discussion
This is the companion method to mergedModelFromBundles:forStoreMetadata:.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hInstance Methods
configurations
Returns all the available configuration names of the receiver.
Return Value
An array containing the available configuration names of the receiver.
Availability
- Available in iOS 3.0 and later.
Declared In
NSManagedObjectModel.hentities
Returns the entities in the receiver.
Return Value
An array containing the entities in the receiver.
Discussion
Entities are instances of NSEntityDescription.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hentitiesByName
Returns the entities of the receiver in a dictionary.
Return Value
The entities of the receiver in a dictionary, where the keys in the dictionary are the names of the corresponding entities.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hentitiesForConfiguration:
Returns the entities of the receiver for a specified configuration.
Parameters
- configuration
The name of a configuration in the receiver.
Return Value
An array containing the entities of the receiver for the configuration specified by configuration.
Availability
- Available in iOS 3.0 and later.
Declared In
NSManagedObjectModel.hentityVersionHashesByName
Returns a dictionary of the version hashes for the entities in the receiver.
Return Value
A dictionary of the version hashes for the entities in the receiver, keyed by entity name.
Discussion
The dictionary of version hash information is used by Core Data to determine schema compatibility.
Availability
- Available in iOS 3.0 and later.
Declared In
NSManagedObjectModel.hfetchRequestFromTemplateWithName:substitutionVariables:
Returns a copy of the fetch request template with the variables substituted by values from the substitutions dictionary.
Parameters
- name
A string containing the name of a fetch request template.
- variables
A dictionary containing key-value pairs where the keys are the names of variables specified in the template; the corresponding values are substituted before the fetch request is returned. The dictionary must provide values for all the variables in the template.
Return Value
A copy of the fetch request template with the variables substituted by values from variables.
Discussion
The variables dictionary must provide values for all the variables. If you want to test for a nil value, use [NSNull null].
This method provides the usual way to bind an “abstractly” defined fetch request template to a concrete fetch. For more details on using this method, see Creating Predicates.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hfetchRequestTemplateForName:
Returns the fetch request with a specified name.
Parameters
- name
A string containing the name of a fetch request template.
Return Value
The fetch request named name.
Discussion
If the template contains substitution variables, you should instead use fetchRequestFromTemplateWithName:substitutionVariables: to create a new fetch request.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hfetchRequestTemplatesByName
Returns a dictionary of the receiver’s fetch request templates.
Return Value
A dictionary of the receiver’s fetch request templates, keyed by name.
Discussion
If the template contains a predicate with substitution variables, you should instead use fetchRequestFromTemplateWithName:substitutionVariables: to create a new fetch request.
Availability
- Available in iOS 3.0 and later.
Declared In
NSManagedObjectModel.hinitWithContentsOfURL:
Initializes the receiver using the model file at the specified URL.
Parameters
- url
An URL object specifying the location of a model file.
Return Value
A managed object model initialized using the file at url.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hisConfiguration:compatibleWithStoreMetadata:
Returns a Boolean value that indicates whether a given configuration in the receiver is compatible with given metadata from a persistent store.
Parameters
- configuration
The name of a configuration in the receiver. Pass
nilto specify no configuration.- metadata
Metadata for a persistent store.
Return Value
YES if the configuration in the receiver specified by configuration is compatible with the store metadata given by metadata, otherwise NO.
Discussion
This method compares the version information in the store metadata with the entity versions of a given configuration. For information on specific differences, use entityVersionHashesByName and perform an entity-by-entity comparison.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hlocalizationDictionary
Returns the localization dictionary of the receiver.
Return Value
The localization dictionary of the receiver.
Discussion
The key-value pattern is described in setLocalizationDictionary:.
Special Considerations
On OS X v10.4, localizationDictionary may return nil until Core Data lazily loads the dictionary for its own purposes (for example, reporting a localized error).
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hsetEntities:
Sets the entities array of the receiver.
Parameters
- entities
An array of instances of
NSEntityDescription.
Special Considerations
This method raises an exception if the receiver has been used by an object graph manager.
Availability
- Available in iOS 3.0 and later.
Declared In
NSManagedObjectModel.hsetEntities:forConfiguration:
Associates the specified entities with the receiver using the given configuration name.
Parameters
- entities
An array of instances of
NSEntityDescription.- configuration
A name for the configuration.
Special Considerations
This method raises an exception if the receiver has been used by an object graph manager.
Availability
- Available in iOS 3.0 and later.
Declared In
NSManagedObjectModel.hsetFetchRequestTemplate:forName:
Associates the specified fetch request with the receiver using the given name.
Parameters
- fetchRequest
A fetch request, typically containing predicates with variables for substitution.
- name
A string that specifies the name of the fetch request template.
Discussion
For more details on using this method, see Creating Predicates.
Special Considerations
This method raises an exception if the receiver has been used by an object graph manager.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hsetLocalizationDictionary:
Sets the localization dictionary of the receiver.
Parameters
- localizationDictionary
A dictionary containing localized string values for entities, properties, and error strings related to the model. The key and value pattern is described in Table 1.
Discussion
Table 1 describes the key and value pattern for the localization dictionary.
Key |
Value |
Note |
|---|---|---|
"Entity/NonLocalizedEntityName" |
"LocalizedEntityName" |
|
"Property/NonLocalizedPropertyName/Entity/EntityName" |
"LocalizedPropertyName" |
(1) |
"Property/NonLocalizedPropertyName" |
"LocalizedPropertyName" |
|
"ErrorString/NonLocalizedErrorString" |
"LocalizedErrorString" |
(1) For properties in different entities with the same non-localized name but which should have different localized names.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hsetVersionIdentifiers:
Sets the identifiers for the receiver.
Parameters
- identifiers
An array of identifiers for the receiver.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.hversionIdentifiers
Returns the collection of developer-defined version identifiers for the receiver.
Return Value
The collection of developer-defined version identifiers for the receiver. Merged models return the combined collection of identifiers.
Discussion
The Core Data framework does not give models a default identifier, nor does it depend this value at runtime. For models created in Xcode, you set this value in the model inspector.
This value is meant to be used as a debugging hint to help you determine the models that were combined to create a merged model.
Availability
- Available in iOS 3.0 and later.
See Also
Declared In
NSManagedObjectModel.h© 2009 Apple Inc. All Rights Reserved. (Last updated: 2009-05-01)