emberjs/data — pull request #1298 by markprzepiora (2013-09-14 13:13:29+00:00)
Title: ActiveModelSerializer tried to normalize polymorphic relationships even when they were not specified

Trying to a load a record with a polymorphic belongsTo relationship would make everything die in a fire if the relation was blank. Same thing when you had a polymorphic hasMany relationship defined (although I understand that this case in general is still a WIP, it's clear that simply having such a relation defined shouldn't break everything.)