Rails/ActiveRecord has_many through：未保存对象的关联

让我们使用这些类：

class User < ActiveRecord::Base
    has_many :project_participations
    has_many :projects, through: :project_participations, inverse_of: :users
end

class ProjectParticipation < ActiveRecord::Base
    belongs_to :user
    belongs_to :project

    enum role: { member: 0, manager: 1 }
end

class Project < ActiveRecord::Base
    has_many :project_participations
    has_many :users, through: :project_participations, inverse_of: :projects
end

A user可以参加很多projects的角色为member or a manager。连接模型称为ProjectParticipation.

我现在在使用未保存对象上的关联时遇到问题。以下命令的工作方式就像我认为的那样：

# first example

u = User.new
p = Project.new

u.projects << p

u.projects
=> #<ActiveRecord::Associations::CollectionProxy [#<Project id: nil>]>

u.project_participations
=> #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil>]>

到目前为止一切顺利 - AR 创造了ProjectParticipation本身，我可以访问projects of a user with u.projects.

但如果我创建它就不起作用ProjectParticipation我自己：

# second example

u = User.new
pp = ProjectParticipation.new
p = Project.new

pp.project = p # assign project to project_participation

u.project_participations << pp # assign project_participation to user

u.project_participations
=> #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil>]>

u.projects
=> #<ActiveRecord::Associations::CollectionProxy []>

为什么项目是空的？我无法通过以下方式访问这些项目u.projects像以前一样。

但如果我直接查看参与，该项目就会显示：

u.project_participations.map(&:project)
=> [#<Project id: nil>]

它不应该像第一个例子一样直接工作吗：u.projects返回我所有项目不取决于我是否自己创建连接对象？或者我怎样才能让 AR 意识到这一点？

简短回答：不，第二个示例不会像第一个示例那样工作。您必须使用第一个示例的方式直接创建与用户和项目对象的中间关联。

长答案:

在开始之前，我们应该知道如何has_many :through正在处理中ActiveRecord::Base。那么，让我们从调用其关联的方法建设者在这里 https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations/builder/association.rb#L28-L42，在方法结束时返回反射 https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/reflection.rb#L623然后添加反射hash https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/reflection.rb#L11作为缓存这里有键值对 https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/reflection.rb#L33-L35.

现在的问题是，这些关联如何被激活？！？！

这是因为association(name) https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations.rb#L150-L160方法。哪个调用association_class https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/reflection.rb#L440-L445方法，它实际上调用并返回这个常量：Associations::HasManyThroughAssociation https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/reflection.rb#L442，这使得这条线 https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations.rb#L118自动加载active_record/associations/has_many_through_association.rb and 实例化它的实例 https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations/has_many_through_association.rb#L7-L12 here https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations.rb#L155。这是哪里所有者和反思 https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations/association.rb#L27在创建关联以及调用下一个在子类中调用的重置方法时保存ActiveRecord::Associations::CollectionAssociation here https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations/collection_association.rb#L64-L67.

为什么这个重置调用很重要？因为，它设定了@target作为一个数组。这@target是在进行查询时存储所有关联对象的数组，然后在代码中重用它而不是进行新查询时将其用作缓存。这就是为什么打电话user.projects（其中用户和项目保留在数据库中，即调用：user = User.find(1)进而user.projects）将进行数据库查询并再次调用它不会。

所以，当你制作一个reader https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations/collection_association.rb#L29-L37呼吁协会，例如：user.projects, it 调用集合代理 https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations/collection_association.rb#L36，在填充之前@target from load_target https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations/collection_association.rb#L373-L380.

这还只是触及表面。但是，您知道如何使用构建器构建关联（这会创建不同的反映 https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/reflection.rb#L15-L31基于条件）并创建用于读取目标变量中的数据的代理。

tl;dr

第一个和第二个示例之间的区别在于调用它们的关联构建器来创建关联反射的方式（基于宏） https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/reflection.rb#L15-L31、代理和目标实例变量。

第一个例子：

u = User.new
p = Project.new
u.projects << p

u.association(:projects)
#=> ActiveRecord::Associations::HasManyThroughAssociation object
#=> @proxy = #<ActiveRecord::Associations::CollectionProxy [#<Project id: nil, name: nil, created_at: nil, updated_at: nil>]>
#=> @target = [#<Project id: nil, name: nil, created_at: nil, updated_at: nil>]

u.association(:project_participations)
#=> ActiveRecord::Associations::HasManyAssociation object
#=> @proxy = #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil, created_at: nil, updated_at: nil>]>
#=> @target = [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil, created_at: nil, updated_at: nil>]

u.project_participations.first.association(:project)
#=> ActiveRecord::Associations::BelongsToAssociation object
#=> @target = #<Project id: nil, name: nil, created_at: nil, updated_at: nil>

第二个例子：

u = User.new
pp = ProjectParticipation.new
p = Project.new

pp.project = p # assign project to project_participation

u.project_participations << pp # assign project_participation to user

u.association(:projects)
#=> ActiveRecord::Associations::HasManyThroughAssociation object
#=> @proxy = nil
#=> @target = []

u.association(:project_participations)
#=> ActiveRecord::Associations::HasManyAssociation object
#=> @proxy = #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil, created_at: nil, updated_at: nil>
#=> @target = [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil, created_at: nil, updated_at: nil>]

u.project_participations.first.association(:project)
#=> ActiveRecord::Associations::BelongsToAssociation object
#=> @target = #<Project id: nil, name: nil, created_at: nil, updated_at: nil>

没有代理BelongsToAssociation，它刚刚目标和所有者 https://github.com/rails/rails/blob/02d3a253610eaf9c80587913b366e3fa0f56b71f/activerecord/lib/active_record/associations/association.rb#L27.

但是，如果您确实想让第二个示例发挥作用，则只需执行以下操作：

u.association(:projects).instance_variable_set('@target', [p])

And now:

u.projects
#=>  #<ActiveRecord::Associations::CollectionProxy [#<Project id: nil, name: nil, created_at: nil, updated_at: nil>]>

在我看来，这是创建/保存关联的一种非常糟糕的方式。所以，坚持第一个例子本身。