Rails ActiveRecord define database objects within other object

Question

I'm relatively new to rails and very new to database manipulation.

I'm trying to create a class within the database which contains a number of custom objects within it. These custom objects are also to be stored in the database in a separate table. I've managed to set this up as follows

class MyClass < ActiveRecord::Base
  has_many :other_objects, :dependent => destroy
end

class OtherObject < ActiveRecord::Base
  belongs_to :my_class
  attr_accessible :some_stuff...
end

I've created the appropriate database tables and managed to get it working.

Now what I want to do is have (four) particular instances of "OtherObject"s in my class, which can be accessed by some straightforward identifier, something like

test = MyClass.new
...
test.instance_of_other_object.some_attribute = "blahblah"

Such that this updates the database entry for the associated object. What is the best way to go about this?

kristinalim · Accepted Answer · 2013-02-19 01:29:13Z

1

That has_many association sets up MyClass#other_objects (and a bunch of other methods) to allow you to easily work with associated records.

You probably want:

my_class.other_objects.each do |other_object|
  other_object.update_attributes(:foo => 'bar')
end

If you want a direct SQL update, you can use update_all:

my_class.other_objects.update_all(:foo => 'bar')

Update:

If that's the sort of association you need, you may define a belongs_to association:

class MyClass < ActiveRecord::Base
  has_many :other_objects, :dependent => :destroy

  # uses :selected_other_object_id
  belongs_to :selected_other_object, :class_name => "OtherObject"
end

my_class = MyClass.first
my_class.selected_other_object = other_object  # Set the object.
# => #<OtherClass:...>
my_class.selected_other_object_id     # ID has been set.
# => 10
my_class.selected_other_object        # Retrieve the object.
# => #<OtherClass:...>
my_class.selected_other_object.save   # Persist ID and other fields in the DB.

my_class = MyClass.find(my_class.id)  # If you fetch the object again...
# => #<MyClass:...>
my_class.selected_other_object_id     # The ID is still there.
# => 10
my_class.selected_other_object        # You have the ID, you get the object.
# => #<OtherClass:...>

my_class.selected_other_object.foo = "bar"  # Access associated object this way.
another_variable = my_class.selected_other_object  # Or this way.

Remember however that this does not assume that :selected_other_object is a subset of :other_objects.

Also note that the selected_other_object and selected_other_object= methods are already set up when you set up the association, so you don't have to define these yourself.

edited Feb 19, 2013 at 1:29

answered Feb 18, 2013 at 11:48

kristinalim

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6 Comments

Jeff Pole Over a year ago

That's not quite what I'm looking for. I want to get and set a single particular instance of the other_object. So my_class.this_special_object_that_i_can_refer_to_from_this_special_identifier.some_attribute = "whatever"

kristinalim Over a year ago

I updated the answer. It might be closer to what you need now.

Jeff Pole Over a year ago

Yes that works in terms of updating the OtherObject in the database. Thank you. However, :selected_other_object itself is not stored in the db so it needs to be pointed towards an entry in the database whenever it is first used in the application. I haven't yet come up with a neat way of doing this

kristinalim Over a year ago

Please see the update sample code in the answer. The selected_other_object= method that your belongs_to application set up already sets the selected_other_object_id in your MyClass object, and would persist this value if you save your MyClass object.

Jeff Pole Over a year ago

This makes no sense to me, even if it works. If you look at the way I have set up the db tables and at the class definitions, surely an 'OtherObject' belongs to a 'MyClass' object. It is clearly shown here (guides.rubyonrails.org/association_basics.html) that for the belongs-to association, the object that belongs to another should have the id of the object it belongs to, which, in this case, is the other way around. On a separate issue, it also would surely not work if you were to add multiple belongs_to :selected_other_object of the same class type, as I am trying to do.

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Jeff Pole · Accepted Answer · 2013-02-18 14:45:45Z

0

This is not a complete answer but I have come up with something that works for getting the object, but not for setting it.

class MyClass < ActiveRecord::Base
  has_many :other_objects, :dependent => destroy

  def particular_instance
    return OtherObject.find_by_id(self.particular_instance_id)
  end
end

Where my db schema looks like this

create_table "my_classs", :force => true do |t|
  t.integer "particular_instance_id"
end

create_table "other_objects", :force => true do |t|
  t.integer "my_class_id"
  t.string "some_attribute"
end

Update In order to set an attribute of the other_object class the update_attributes method can be used

my_class.particular_instance.update_attributes(:some_attribute => "blah")

edited Feb 18, 2013 at 14:45

answered Feb 18, 2013 at 12:04

Jeff Pole

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Comments

sjain · Accepted Answer · 2013-02-18 14:50:53Z

0

Use object.dup now for rails 3.1 and on.

a = MyClass.first # finds the first instance of the model "MyClass"  

b = a.dup # duplicates the record.

c = a.dup # duplicates the record again.  

b.field_name = "blahblah"

c.fielf_name = "blahblah"

b.save # saves the record into the database for object b.

c.save # saves the record into the database for object c.

If you look into your database, you can see that a new record has been created. It is identical to the first record excepted that it has a new id.

Also check Duplicate a model for more information about it.

answered Feb 18, 2013 at 14:50

sjain

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