refactor(Container): move defineRelations method to Mapper

[stalniy]

No description provided.

[codecov-io]

Current coverage is 93.30%

Merging #345 into master will decrease coverage by <.01%

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[jmdobry]

Looks good, though after looking through the changes, I'm not sure I fully understand the reasoning behind them. Can you comment here (or update the description of the PR) with a description of the purpose of these changes?

[stalniy]

@jmdobry
Notes:

• relations.js was added as a single entry point for all relations. So, then you safely can import Relation class and be sure that all relation types are included as well. Don't want to keep this logic inside decorators.js as this has nothing to do with decorators
• it's useful to have static methods on Relation class as we can hide internal implementation of all relation types
• I want to hide all relation logic inside Relation class
• defineRelations was moved from Container to Mapper because relation definitions is something what relates more to Mapper than to Container
• also it's better to replace if-s logic with polymorphism. As a result in future, when you add a new relation type, you won't need to change code inside Mapper.js.

[jmdobry]

perfect