Create if record does not exist

Question

I have 3 models in my rails app ``` class Contact < ActiveRecord::Base belongs_to :survey, counter_cache: :contact_count belongs_to :voter has_many :contact_attempts end class Survey < ActiveRecord::Base has_many :questions has_many :contacts end class Voter < ActiveRecord::Base has_many :contacts end ``` the Contact consists of the voter_id and a survey_id. The Logic of my app is that a there can only be one contact for a voter in any given survey. right now I am using the following code to enforce this logic. I query the contacts table for records matching the given voter_id and survey_id. if does not exist then it is created. otherwise it does nothing. ``` if !Contact.exists?(:survey_id => survey, :voter_id => voter) c = Contact.new c.survey_id = survey c.voter_id = voter c.save end ``` Obviously this requires a select and a insert query to create 1 potential contact. When I am adding potentially thousands of contacts at once. Right now I'm using Resque to allow this run in the bac…

Accepted Answer

You should add first a database index to force this condition at the lowest level as possible:

```
add_index :contacts, [:voter_id, :survey_id], unique: true
```

Then you should add an uniqueness validation at an ActiveRecord level:

```
validates_uniqueness_of :voter_id, scope: [:survey_id]
```

Then `contact.save` will return `false` if a contact exists for a specified voter and survey.

UPDATE: If you create the index, then the uniqueness validation will run pretty fast.

Create if record does not exist

Solution

Problem