How do I elegantly handle two-sided relations with Rails translations?

Question

I have a family_tree and someone can add their relatives to the tree.

So what happens is there is a membership record created for each family_tree entry.

However, if a Son adds a Dad, we should be able to update the family tree of the Dad to add the "Son" to the tree in the view. What's the best Rails way to approach this? I know Rails does a lot of translations natively, and pluralizations, etc. Anyway for me to leverage that for what I want to do?

Also, what is the class/module that handles that stuff again? ActiveSupport?

This is my User model:

# == Schema Information
#
# Table name: users
#
#  id                     :integer          not null, primary key
#  email                  :string(255)      default(""), not null
#  encrypted_password     :string(255)      default(""), not null
#  reset_password_token   :string(255)
#  reset_password_sent_at :datetime
#  remember_created_at    :datetime
#  sign_in_count          :integer          default(0), not null
#  current_sign_in_at     :datetime
#  last_sign_in_at        :datetime
#  current_sign_in_ip     :string(255)
#  last_sign_in_ip        :string(255)
#  created_at             :datetime
#  updated_at             :datetime
#  name                   :string(255)
#  confirmation_token     :string(255)
#  confirmed_at           :datetime
#  confirmation_sent_at   :datetime
#  unconfirmed_email      :string(255)
#  invitation_relation    :string(255)
#  avatar                 :string(255)
#
class User < ActiveRecord::Base
  has_one :family_tree, dependent: :destroy
  has_many :memberships, dependent: :destroy
  has_many :nodes, dependent: :destroy
  has_many :participants, dependent: :destroy    
end

FamilyTree.rb

# == Schema Information
#
# Table name: family_trees
#
#  id         :integer          not null, primary key
#  name       :string(255)
#  user_id    :integer
#  created_at :datetime
#  updated_at :datetime
#

class FamilyTree < ActiveRecord::Base
  belongs_to :user
  has_many :memberships, dependent: :destroy
  has_many :members, through: :memberships, source: :user, dependent: :destroy
  has_many :nodes, dependent: :destroy
end

Membership.rb:

# == Schema Information
#
# Table name: memberships
#
#  id             :integer          not null, primary key
#  family_tree_id :integer
#  user_id        :integer
#  created_at     :datetime
#  updated_at     :datetime
#  relation       :string(255)
#

class Membership < ActiveRecord::Base    
  belongs_to :family_tree
  belongs_to :user      
end

Node.rb

# == Schema Information
#
# Table name: nodes
#
#  id             :integer          not null, primary key
#  name           :string(255)
#  family_tree_id :integer
#  user_id        :integer
#  media_id       :integer
#  media_type     :string(255)
#  created_at     :datetime
#  updated_at     :datetime
#  circa          :datetime
#  is_comment     :boolean
#

class Node < ActiveRecord::Base
  belongs_to :family_tree
  belongs_to :user
  belongs_to :media, polymorphic: true, dependent: :destroy
  has_many :comments, dependent: :destroy
  has_many :node_comments, dependent: :destroy    
end

My _tree.html.erb looks like this (truncated for brevity):

      <li class="tree-item-name"><a href="#">Great Grandparents</a>
        <ul>
          <li><% if relative.humanize == "Great Grandfather" || relative.humanize == "Great Grandmother" %>
            <%= link_to image_tag(membership.user.avatar.url, size: "48x48", :class => "img-circle") , family_tree_path(membership.user.family_tree), :target => '_blank' %>
                <%= link_to membership.user.name, family_tree_path(membership.user.family_tree), :target => '_blank'%>
              <% else %>
              None added yet, add them <%= link_to 'here', "#" , class: 'btn invite popupbox','data-popup' => 'invite_friend' %>
            <% end %>
          </li>
        </ul>
      </li>
      <li class="tree-item-name"><a href="#">Grandparents</a>
        <ul>
          <li><% if relative.humanize == "Grandfather" || relative.humanize == "Grandmother" %>
            <%= link_to image_tag(membership.user.avatar.url, size: "48x48", :class => "img-circle") , family_tree_path(membership.user.family_tree), :target => '_blank' %>
                <%= link_to membership.user.name, family_tree_path(membership.user.family_tree), :target => '_blank' %>
              <% else %>
              None added yet, add them <%= link_to 'here', "#" , class: 'btn invite popupbox','data-popup' => 'invite_friend' %>
            <% end %>
          </li>
        </ul>
      </li>

Answer 1

I would use the same relations you defined in the question, except this part:

class Membership < ActiveRecord::Base    
  belongs_to :family_tree
  belongs_to :user_one, class_name: 'User'
  belongs_to :user_two, class_name: 'User' # I actually have no idea how to call them!
  belongs_to :relation # to hold values likes 'Son', 'Dad', etc.
  # The model Relation would be as simple as a name and internal reference, nothing else.
  # (internal_reference is here to solve the translation problems and other stuff you will understand with the following code)

With a callback after_create to reverse the membership created:

def create_reverse_membership
  user_one_is_female = user_one.gender == 'female'
  user_two_is_female = user_two.gender == 'female'
  son_or_daughter = user_one_is_female ? :daughter : :son
  father_or_mother = user_two_is_female ? :mother : :father

  case relation.internal_reference.to_sym
  when :son
    relation = Relation.find_by_internal_reference(father_or_mother)
    membership = Membership.where(relation_id: relation.id, user_one: user_two.id, user_two: user_one.id).first
    if membership.present?
      # This means the reverse membership already exists, do not call Membership.create here because it would cause and endless loop with the callback
    else
      membership = Membership.create(relation_id: relation.id, user_one: user_two, user_two: user_one)
    end
  when :father
    # almost same logic but with `son_or_daughter`
  when :mother
  else

  end
end

English not being my native language, this code probably lacks of consistency (coherence, logic).

Hope this helps!