Relation refactoring: Split Repository interface into different interfaces

[dhmlau]

Description / Steps to reproduce / Feature proposal

See #1194 (comment) for background. This tasks aims to break up the Repository interface into different interfaces based on the persistence function type and refactor the repository interfaces which extend from it to use those interfaces via a Mixin.

Acceptance Criteria

• Split Repository interface into different interfaces based on the persistence function type i.e. LookupRepository interface to have all the Retrieval methods, WriteRepository (I'm sure there is a better name), would have the create methods, MutationRepository might have the update and related methods (this might fall under the previous one), and DestroyRepository for deletes. A good list from @raymondfeng:

1. QueryRepository - read-based operations (find, findOne, count, ...)
2. MutationRepository - write-based operations (create, update, delete, ...)
3. CrudRepository (QueryRepository + MutationRepository)
4. EntityRepository (CrudRepository + findById, updateById, deleteById, ...)

• Explore the use of a Mixin for a smart way of sharing the implementation bits from the different repositories (for e.g. how can HasManyEntityCrudRepository inherit the needed CRUD methods from the interfaces mentioned above?).

• tests / docs

[jannyHou]

from @raymondfeng
different ways to split the repository interfaces:
query vs mutation
PK based vs KV based

follow-up discussion:
know the nature of a method at run-time
run-time access control for certain methods(e.g. mutation methods)

We need more time to discuss. Revisit it next time.

[jannyHou]

@strongloop/sq-lb-apex @hacksparrow @raymondfeng @bajtos Any updates on this story?

[b-admike]

@jannyHou Sorry, I need to work with @raymondfeng to update the acceptance criteria. I'll have it ready for our next estimation meeting.

[raymondfeng]

Here are some candidates I see:

1. QueryRepository - read-based operations (find, findOne, count, ...)
2. MutationRepository - write-based operations (create, update, delete, ...)
3. CrudRepository (QueryRepository + MutationRepository)
4. EntityRepository (CrudRepository + findById, updateById, deleteById, ...)