[EPIC] Composite primary/foreign keys

See https://github.com/strongloop/loopback/issues/2080 for the original discussion in LB 2.x/3.x.

LoopBack should support models using composite keys, where a primary key or a foreign key is composed from multiple properties.

Let's consider the following models as an example:

// A (GitHub) repository is identified by the pair [user, name]
// and has many issues
@model
class Repository {
  @property({id: true})
  user: string;

  @property({id: true}
  name: string;

  @hasMany(() => Issue)
  issues: Issue[];
}

// An issue belongs to a repository
@model
class Issue {
  // how to specify @belongsTo relation?
}

Functional areas we need to cover:

• How to define a model with a composite primary key (e.g. Repository above)?
• Connectors must understand composite ids and convert them to a concept the target database understands. SQL is easy because it supports composite primary keys OOTB, NoSQL like MongoDB and Cloudant can be tricky.
  • create a new model
  • find a model by id
  • update (patch/replace) a model by id
  • delete a model by id
  • (etc.)
• Autoupdate/automigrate need to support composite primary keys too
• How to define composite foreign key in a model belonging to another model (e.g. Issue above)
• Relation repositories must work with composite keys
• How to expose a model with a composite primary key via REST API?
• How to expose a relation using a composite foreign key via REST API?
• Controller templates need to produce code that works with composite primary/foreign keys.

Bắt đầu với cuộc thảo luận được liên kết về LB 2.x/3.x và ví dụ về model Repository/Issue. Phạm vi công việc bao gồm các định nghĩa model, connector, automigrate/autoupdate, các relation repository, việc expose REST và các controller template; công việc được coi là hoàn tất khi đã thống nhất việc hỗ trợ cho từng operation được liệt kê và có một kế hoạch triển khai có thể kiểm thử.