This issue duplicates another issue.
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4.1.* Model save updates the fields but not hasOne associations.
[4.1.*] Model save updates the fields but not hasOne associations.
Ext version tested:
Save a model that has a hasOne association and notice that is does not get updated based on the servers result.
I attached a folder which is the basically the MVC example that comes with 4.1.1. If you run it you can notice that when you click on the songs list the Blues at the Sunrise name gets updated but the type does not.
Believe this is a duplicate