title: iOS Core Data Merge Policy - NSMergeByPropertyStoreTrumpMergePolicy
body: <p>I read apple developer document for core data merge policy about NSMergeByPropertyStoreTrumpMergePolicy.</p>
  <p><a href="https://developer.apple.com/documentation/coredata/nsmergebypropertystoretrumpmergepolicy" rel="noreferrer">https://developer.apple.com/documentation/coredata/nsmergebypropertystoretrumpmergepolicy</a></p>
  <p>In document,</p>
  <blockquote>
  <p>A policy that merges conflicts between the persistent store's version of the object and the current in-memory version by individual property, with the in-memory changes trumping external changes.</p>
  <p>The merge occurs by individual property. For properties that have been changed in both the external source and in memory, the external changes trump the in-memory ones.</p>
  </blockquote>
  <p>I can't understand what 'individual property', 'in-memory changes trumping external change' and 'the external changes trump the in-memory ones' means.</p>
  <p>I think 'in-memory changes' is current context to save. And 'external change' is persistent store changed by other context before. Is it right?</p>
  <p>Thanks for your help.</p>