Category Archives: Rails

Cool stuff you can do with rails has_one

Over the past three months has_one has shown itself to be the answer to questions that I was struggling to get an elegent answer for.

The two cases I found were:

1. Specifying a belongs_to :through
2. Accessing one item of a list of related records

The second case ends up expanding to be anytime you want to have a named scope that returns a single record.

Specifying a belongs_to :through with has_one

Say you have a model of the form:

You can see that we’ve used the standard has_many and belongs_to methods to model the relationships between the entities. We’ve also thrown in a has_many through to help model the fact that an organization has_many employees. Conceptually we also want to be able to specify the inverse of this relationship on the Employee. Something like:

The way to implement the belongs_to :through concept is to use has_one :through

Accessing one item of a list of related records

There can often be a desire to specify a scope that returns a single record. This isn’t possible in rails, as scopes by definition are chainable and will return subclasses of ActiveRecord::Relation, which is a list like object.

The way to do this is to setup a has_one with a block specifying a scope. For example:

has_one :head_of_department, -> { where role: ‘department_head’ }, class_name: ‘Employee’

For bonus points the proc and conditions can also be parameterised, so we can end up with a has_one relationship with a parameter. To see the coolness of this we can extend the model a little bit to include
a many-to-many entity and a has_many through relationship.

With the above model it would be handy to have an easy way of accessing the ProjectDepartment easily when you are working with projects. has_one to the rescue.

So with our collection of has ones all together, we end up with code something like:

 

has_one is a powerful tool to have in your rails modelling. Take advantage of it to give some really nice clean looking code.