Support for the Many-to-One association

[mereghost]

Albeit we had belongs_to already it didn't really do anything. This PR implements it as far as we can.

class AuthorRepository < Hanami::Repository
  associations do
    has_many :books
  end
end

class BookRepository < Hanami::Repository
  associations do
    belongs_to :author  #the relation name is inferred by pluralization
  end

  # the authors relation is aliased to author on this scope
  def find_with_author(id)
    aggregate(:author).where(id: id).as(Book).one
  end

  def author_for(book)
    assoc(:author, book).one
  end
end

Note that we don't support the creation of a parent record from a child one.

@hanami/core anything missing?

PS: The changes on the ConstraintViolationErrors were mainly due to things being broken on my machine and that... well it makes sense. ;)

[mereghost]

Rebased over develop so I can pick the symbolize changes. =D

[AlfonsoUceda]

@mereghost 👍 It works perfect ;)

I want to share with you one thing I've found if you put a wrong relation name.

• In belongs_to (belongs_to :autho):

 bookshelf git:(master) ✗ be hanami console
bundler: failed to load command: hanami (/Users/alfonsouceda/.gem/ruby/2.3.3/bin/hanami)
Hanami::Model::Error: key not found: :autho

• In has_many (has_many :book):

 bookshelf git:(master) ✗ be hanami console
irb(main):001:0> authors = AuthorRepository.new
ROM::Registry::ElementNotFoundError: :book doesn't exist in ROM::RelationRegistry registry

Notice the difference, can we unify them? I think we don't need to do it here, we can do it in another PR.

What do you think @jodosha?

[jodosha]

@AlfonsoUceda I'd unify these error conditions when we will implement other associations as well. /cc @mereghost

[jodosha]

@mereghost I'm gonna merge this now. Please document this new feature on the Guides, using this PR: hanami/hanami.github.io#334

[sovetnik]

i'am on belongs_to branch found this behaviour:
Assume we have two models and relations between them:
User has_many Projects
Project belongs_to User

When we aggregate users from projects, aggregated entities (users) becomes ROM::Struct like #<ROM::Struct[User]… instead a User instance?

class UserRepository < Hanami::Repository
  associations do
    has_many :projects
  end
end

class ProjectRepository < Hanami::Repository
  associations do
    belongs_to :user
  end
end

> UserRepository.new.aggregate(:projects).as(User).one
=> #<User:0x007faa23217aa8
 @attributes=
  {:id=>1, :name=>"Oleg", :nickname=>"sovetnik", :created_at=>2017-05-13 12:26:22 UTC, :updated_at=>2017-05-13 12:26:22 UTC, :projects=>[
#<Project:0x007faa22e183a8 @attributes={:id=>2, :user_id=>1, :head_id=>nil, :name=>"Chaos"}>,  
#<Project:0x007faa22e2b570 @attributes={:id=>3, :user_id=>1, :head_id=>nil, :name=>"Discord"}>
]}>

> ProjectRepository.new.aggregate(:user).as(Project).to_a
=> [#<Project:0x007faa224a4d58
  @attributes=
   {:id=>2, :user_id=>1, :head_id=>nil, :name=>"Chaos", :user=>#<ROM::Struct[User] id=1 name="Oleg" nickname="sovetnik" created_at=2017-05-13 12:26:22 UTC updated_at=2017-05-13 12:26:22 UTC project_id=3>}>,
 #<Project:0x007faa224ad7c8
  @attributes=
   {:id=>3, :user_id=>1, :head_id=>nil, :name=>"Discord", :user=>#<ROM::Struct[User] id=1 name="Oleg" nickname="sovetnik" created_at=2017-05-13 12:26:22 UTC updated_at=2017-05-13 12:26:22 UTC project_id=3>}>]