Index on multiple columns in Ruby on Rails

Question

I\'m implementing functionality to track which articles a user has read.

  create_table \"article\", :force => true do |t|
    t.string   \"title\"
    t.         


        

Answer 1

The order does matter in indexing.

1. Put the most selective field first, i.e. the field that narrows down the number of rows fastest.
2. The index will only be used insofar as you use its columns in sequence starting at the beginning. i.e. if you index on [:user_id, :article_id], you can perform a fast query on user_id or user_id AND article_id, but NOT on article_id.

Your migration add_index line should look something like this:

add_index :user_views, [:user_id, :article_id]

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Question regarding 'unique' option

An easy way to do this in Rails is to use validates in your model with scoped uniqueness as follows (documentation):

validates :user, uniqueness: { scope: :article }

Answer 2

Just a warning about checking uniqueness at validation time vs. on index: the latter is done by database while the primer is done by the model. Since there might be several concurrent instances of a model running at the same time, the validation is subject to race conditions, which means it might fail to detect duplicates in some cases (eg. submit twice the same form at the exact same time).