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# NSEntityMigrationPolicy

A policy instance that customizes the migration process for an entity mapping.

```
class NSEntityMigrationPolicy
```

## Overview

You set the policy for an entity mapping by passing the name of the migration policy class as the argument to [`entityMigrationPolicyClassName`](/documentation/CoreData/NSEntityMapping/entityMigrationPolicyClassName). Typically, you specify the name in the Xcode mapping model editor.

## Topics

### Customizing Stages of the Mapping Life Cycle

[`begin(_:with:)`](/documentation/CoreData/NSEntityMigrationPolicy/begin(_:with:))

Sets up state information before the start of a given entity mapping.

[`createDestinationInstances(forSource:in:manager:)`](/documentation/CoreData/NSEntityMigrationPolicy/createDestinationInstances(forSource:in:manager:))

Creates the destination instance(s) for a given source instance.

[`endInstanceCreation(forMapping:manager:)`](/documentation/CoreData/NSEntityMigrationPolicy/endInstanceCreation(forMapping:manager:))

Indicates the end of the instance creation stage for the specified entity mapping, and the precursor to the next migration stage.

[`createRelationships(forDestination:in:manager:)`](/documentation/CoreData/NSEntityMigrationPolicy/createRelationships(forDestination:in:manager:))

Constructs the relationships between the newly-created destination instances.

[`endRelationshipCreation(forMapping:manager:)`](/documentation/CoreData/NSEntityMigrationPolicy/endRelationshipCreation(forMapping:manager:))

Indicates the end of the relationship creation stage for the specified entity mapping.

[`performCustomValidation(forMapping:manager:)`](/documentation/CoreData/NSEntityMigrationPolicy/performCustomValidation(forMapping:manager:))

Provides the option to perform custom validation on migrated objects during the validation stage of the entity migration policy.

[`end(_:manager:)`](/documentation/CoreData/NSEntityMigrationPolicy/end(_:manager:))

Performs cleanup at the end of the migration, from any phase of the mapping.

### Constants

[`NSMigrationManagerKey`](/documentation/CoreData/NSMigrationManagerKey)

Key for the migration manager.

[`NSMigrationSourceObjectKey`](/documentation/CoreData/NSMigrationSourceObjectKey)

Key for the source object.

[`NSMigrationDestinationObjectKey`](/documentation/CoreData/NSMigrationDestinationObjectKey)

Key for the destination object.

[`NSMigrationEntityMappingKey`](/documentation/CoreData/NSMigrationEntityMappingKey)

Key for the entity mapping object.

[`NSMigrationPropertyMappingKey`](/documentation/CoreData/NSMigrationPropertyMappingKey)

Key for the property mapping object.

[`NSMigrationEntityPolicyKey`](/documentation/CoreData/NSMigrationEntityPolicyKey)

Key for the entity migration policy object.


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