Fix exception when using a resource with a composite primary/foreign key association

[filipegiusti]

Fixes #3334

Fix NoMethodError: undefined method to_sym' for an instance of Array` raised by this line when an association uses a composite primary key/foreign key.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

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