问题
I'm writing a mentorship program for our church in rails (im still farily new to rails)..
And i need to model this..
contact
has_one :father, :class_name => "Contact"
has_one :mother, :class_name => "Contact"
has_many :children, :class_name => "Contact"
has_many :siblings, :through <Mother and Father>, :source => :children
So basically an objects "siblings" needs to map all the children from both the father and mother not including the object itself..
Is this possible?
Thanks
Daniel
回答1:
It's funny how questions that appear simple can have complex answers. In this case, implementing the reflexive parent/child relationship is fairly simple, but adding the father/mother and siblings relationships creates a few twists.
To start, we create tables to hold the parent-child relationships. Relationship has two foreign keys, both pointing at Contact:
create_table :contacts do |t|
t.string :name
end
create_table :relationships do |t|
t.integer :contact_id
t.integer :relation_id
t.string :relation_type
end
In the Relationship model we point the father and mother back to Contact:
class Relationship < ActiveRecord::Base
belongs_to :contact
belongs_to :father, :foreign_key => :relation_id, :class_name => "Contact",
:conditions => { :relationships => { :relation_type => 'father'}}
belongs_to :mother, :foreign_key => :relation_id, :class_name => "Contact",
:conditions => { :relationships => { :relation_type => 'mother'}}
end
and define the inverse associations in Contact:
class Contact < ActiveRecord::Base
has_many :relationships, :dependent => :destroy
has_one :father, :through => :relationships
has_one :mother, :through => :relationships
end
Now a relationship can be created:
@bart = Contact.create(:name=>"Bart")
@homer = Contact.create(:name=>"Homer")
@bart.relationships.build(:relation_type=>"father",:father=>@homer)
@bart.save!
@bart.father.should == @homer
This is not so great, what we really want is to build the relationship in a single call:
class Contact < ActiveRecord::Base
def build_father(father)
relationships.build(:father=>father,:relation_type=>'father')
end
end
so we can do:
@bart.build_father(@homer)
@bart.save!
To find the children of a Contact, add a scope to Contact and (for convenience) an instance method:
scope :children, lambda { |contact| joins(:relationships).\
where(:relationships => { :relation_type => ['father','mother']}) }
def children
self.class.children(self)
end
Contact.children(@homer) # => [Contact name: "Bart")]
@homer.children # => [Contact name: "Bart")]
Siblings are the tricky part. We can leverage the Contact.children method and manipulate the results:
def siblings
((self.father ? self.father.children : []) +
(self.mother ? self.mother.children : [])
).uniq - [self]
end
This is non-optimal, since father.children and mother.children will overlap (thus the need for uniq), and could be done more efficiently by working out the necessary SQL (left as an exercise :)), but keeping in mind that self.father.children and self.mother.children won't overlap in the case of half-siblings (same father, different mother), and a Contact might not have a father or a mother.
Here are the complete models and some specs:
# app/models/contact.rb
class Contact < ActiveRecord::Base
has_many :relationships, :dependent => :destroy
has_one :father, :through => :relationships
has_one :mother, :through => :relationships
scope :children, lambda { |contact| joins(:relationships).\
where(:relationships => { :relation_type => ['father','mother']}) }
def build_father(father)
# TODO figure out how to get ActiveRecord to create this method for us
# TODO failing that, figure out how to build father without passing in relation_type
relationships.build(:father=>father,:relation_type=>'father')
end
def build_mother(mother)
relationships.build(:mother=>mother,:relation_type=>'mother')
end
def children
self.class.children(self)
end
def siblings
((self.father ? self.father.children : []) +
(self.mother ? self.mother.children : [])
).uniq - [self]
end
end
# app/models/relationship.rb
class Relationship < ActiveRecord::Base
belongs_to :contact
belongs_to :father, :foreign_key => :relation_id, :class_name => "Contact",
:conditions => { :relationships => { :relation_type => 'father'}}
belongs_to :mother, :foreign_key => :relation_id, :class_name => "Contact",
:conditions => { :relationships => { :relation_type => 'mother'}}
end
# spec/models/contact.rb
require 'spec_helper'
describe Contact do
before(:each) do
@bart = Contact.create(:name=>"Bart")
@homer = Contact.create(:name=>"Homer")
@marge = Contact.create(:name=>"Marge")
@lisa = Contact.create(:name=>"Lisa")
end
it "has a father" do
@bart.relationships.build(:relation_type=>"father",:father=>@homer)
@bart.save!
@bart.father.should == @homer
@bart.mother.should be_nil
end
it "can build_father" do
@bart.build_father(@homer)
@bart.save!
@bart.father.should == @homer
end
it "has a mother" do
@bart.relationships.build(:relation_type=>"mother",:father=>@marge)
@bart.save!
@bart.mother.should == @marge
@bart.father.should be_nil
end
it "can build_mother" do
@bart.build_mother(@marge)
@bart.save!
@bart.mother.should == @marge
end
it "has children" do
@bart.build_father(@homer)
@bart.build_mother(@marge)
@bart.save!
Contact.children(@homer).should include(@bart)
Contact.children(@marge).should include(@bart)
@homer.children.should include(@bart)
@marge.children.should include(@bart)
end
it "has siblings" do
@bart.build_father(@homer)
@bart.build_mother(@marge)
@bart.save!
@lisa.build_father(@homer)
@lisa.build_mother(@marge)
@lisa.save!
@bart.siblings.should == [@lisa]
@lisa.siblings.should == [@bart]
@bart.siblings.should_not include(@bart)
@lisa.siblings.should_not include(@lisa)
end
it "doesn't choke on nil father/mother" do
@bart.siblings.should be_empty
end
end
回答2:
I totally agree with zetetic. The question looks far more simpler then the answer and there is little we could do about it. I'll add my 20c though.
Tables:
create_table :contacts do |t|
t.string :name
t.string :gender
end
create_table :relations, :id => false do |t|
t.integer :parent_id
t.integer :child_id
end
Table relations does not have corresponding model.
class Contact < ActiveRecord::Base
has_and_belongs_to_many :parents,
:class_name => 'Contact',
:join_table => 'relations',
:foreign_key => 'child_id',
:association_foreign_key => 'parent_id'
has_and_belongs_to_many :children,
:class_name => 'Contact',
:join_table => 'relations',
:foreign_key => 'parent_id',
:association_foreign_key => 'child_id'
def siblings
result = self.parents.reduce [] {|children, p| children.concat p.children}
result.uniq.reject {|c| c == self}
end
def father
parents.where(:gender => 'm').first
end
def mother
parents.where(:gender => 'f').first
end
end
Now we have regular Rails assosiations. So we can
alice.parents << bob
alice.save
bob.chidren << cindy
bob.save
alice.parents.create(Contact.create(:name => 'Teresa', :gender => 'f')
and all stuff like that.
回答3:
has_and_belongs_to_many :parents,
:class_name => 'Contact',
:join_table => 'relations',
:foreign_key => 'child_id',
:association_foreign_key => 'parent_id',
:delete_sql = 'DELETE FROM relations WHERE child_id = #{id}'
has_and_belongs_to_many :children,
:class_name => 'Contact',
:join_table => 'relations',
:foreign_key => 'parent_id',
:association_foreign_key => 'child_id',
:delete_sql = 'DELETE FROM relations WHERE parent_id = #{id}'
I used this example but had to add the :delete_sql to clean up the relations records. At first I used double quotes around the string but found that caused errors. Switching to single quotes worked.
来源：https://stackoverflow.com/questions/4987541/has-many-through-multiple-has-one-relationships