[EPIC] Model Relations (4.0 GA)

[dhmlau]

Description / Steps to reproduce / Feature proposal

This is the Model Relations epic for GA release, and includes the following the relation types:

• BelongsTo [Relation] Support BelongsTo relation #1361

Stretch goals

• CLI tooling (lb4 relation) [CLI] add lb4 relation command #1359
• Split Repository interface into different interfaces Relation refactoring: Split Repository interface into different interfaces #1356
• HasOne [Relation] Supports HasOne relation #1422 (moved to stretch goals by @bajtos @raymondfeng and @virkt25 on 2018-08-28)

Out of scope

• HasManyThrough
• HasAndBelongsToMany
• Polymorphic
• EmbedsOne
• EmbedsMany
• ReferencesMany

Note: The last four relation types above had lots of issues in LB3. We need to rethink the design.

What we have for DP3

• HasMany