Rails 3. How to remove duplicate associated records from query result?

Question

I have @invoices, which contains all invoices related to a particular scheduled course. ``` scheduled_course has_many :invoices, through => :enrollments invoices belongs_to :scheduled_courses @scheduled_course = ScheduledCourse.find(params[:id]) @invoices = @scheduled_course.invoices ``` Now, some invoices might be duplicated. For example, `@invoices - @scheduled_course.invoices` might output... ``` => [#, student_id: nil, due_date: "2012-04-02">, #, student_id: nil, due_date: "2012-04-02">, #

Accepted Answer

You can pass the `:uniq` option to has many:

```
has_many :invoices, :through => :enrollments, :uniq => true
```

Here's the description from the RoR docs for this option:

If true, duplicates will be omitted from the collection. Useful in conjunction with :through.

Also, to avoid duplicates from entering the DB in the first place, you may be able to use a unique DB index, which may look something like this:

```
add_index :enrollments, [:invoice_id, :scheduled_course_id], :unique => true
```

Rails 3. How to remove duplicate associated records from query result?

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