title: Multiple Pass Core Data Migration In Chunks With iOS
body: <p>I'm trying to migrate an iPhone/iPad sqlite store of about 11Mb. It has around thirty different entities, all quite heavily interrelated. </p>
  
  <p>On a 3GS, it takes quite some time. Even a lightweight migration takes around four minutes.</p>
  
  <p>I guess this is because all the relationships are held in memory.</p>
  
  <p>Reading the Apple migration guide, it says that for large datasets, one approach is to do multiple passes using different mapping models.</p>
  
  <p>All well and good. However, it seems that you can't do this if the entities in one mapping model are related to those in another. In fact, a compilation error is generated along the lines of  "can't auto generate value expression with nil mapping name" in the Mapping Model xcmappingmodel file.</p>
  
  <p>Any tips much appreciated.</p>
  
  <p>Many thanks,</p>
  
  <p>Max</p>