Generate primary keys without creating the record - sql

Question

I want to be able to do something like
#foo = MyClass.new
5.times do
#foo.things.build
end
But my #foo needs to have a primary key for this to work, Soo what is the best way to generate primary keys without creating the object?
The purpose for this is to be able to use nested forms more easely
form_builder.fields_for :things do ...

Answer 1

I believe the OP is asking for how to initialize a view action property for use in new action for a standard Rails resource. At this point, there is no ID for the main parent. The solution is simple:
The model:
class ParentObject < ActiveRecord::Base
# the child model in this example is called child_objects
has_many :child_objects, :dependent => :destroy
accepts_nested_attributes_for :child_objects
The controller action for new:
#object = Object.new :example_field => "my field"
#object.child_objects.build :name => "value_1" # pretending that name is a field
#object.child_objects.build :name => "value_2"
Then, in the view:
= form_for(#object) do |f| # top level Object
= f.label :example_field
= f.text_field :example_field
=# the next line loops twice in this example
= f.fields_for :child_objects do |child|
= child.label :name
= child.text_field :name
There is also a good gem called nested_form written by Ryan Bates (https://github.com/ryanb/nested_form) which may help you with the rest of the CRUD operations.

Answer 2

What you probable want is NestedAttributes
Nested attributes allow you to save attributes on associated records through the parent. By default nested attribute updating is turned off, you can enable it using the accepts_nested_attributes_for class method. When you enable nested attributes an attribute writer is defined on the model.
The implementation is different between each ORM, here is for sequel and ActiveRecord
NOTE: Full tutorial also available at Nerdgem
Sequel impementation
Imagine there is a Project class that has many tasks
class Project < Sequel::Model
one_to_many :tasks
end
class Task < Sequel::Model
many_to_one :project
end
To enable the nested attributes you will need include two plugins for the Project class
Sequel::Plugins::NestedAttributes: allows you to create, update, and delete associated objects directly by calling a method on the current object. Nested attributes are defined using the nested_attributes class method:
Sequel::Plugins::InstanceHooks: which is a dependency of NestedAttributes
You can find really good doc on the plugin site
Project.plugin :instance_hooks
Project.plugin :nested_attributes
After that is done you can call the nested_attributes method on the desired class
Project.nested_attributes :tasks
Now you can do this
p = Project.new(:title=>'Project')
p.tasks_attributes = [{:title=>'First Task'}, {:title=>'Second Task'}]
puts p.tasks.inspect
# It will output this
# [#<Task #values={:title=>"First Task"}>, #<Task #values={:title=>"Second Task"}>]
When you save the project it will save both the project and the tasks.
If you can even to edit many tasks at the same.
ActiveRecord implementation
Here is how to use it.
Imagine there is a Project class that has many tasks
Project.rb
class Project < ActiveRecord::Base
attr_accessible :title
has_many :tasks
accepts_nested_attributes_for :tasks
end
Task.rb
class Tasks < ActiveRecord::Base
attr_accessible :title, :project_id
belongs_to :project
end
Now you can do this.
p = Project.new
p.tasks_attributes=[{title: "First Task"}]
p.things
# Would output this
#=> [#<Thing id: nil, title: "First Task", created_at: nil, updated_at: nil, bar_id: nil>]
p.save
When you save the project it will save both the project and the tasks.
If you want to edit many project tasks at the same time you can to this
p.tasks_attributes=[{title: "First Task"},{title: "Second Task"}]
NOTE: there is also a Railscasts that can help you out with nested forms. Orginal Railscast, Revised Railscast