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Fix for correcting "changed" after reset [ #2676 ]
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Add specs for #2700
Fix bug with eager_loads two objects from different classes, which inherited from one class.
Add tests for eager_loads two different relations, which classes inherited from another one class (has_many and has_one types of relations).
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Return 0 instead of nil on sum with no matches.

[ fix #2678 ]
Force to p327