ActiveRecord, what does “index: true” mean?

Question

I'm writing a migration that involves a foreign key. Looking at my colleagues code, I see that he has added the line: t.reference :tablename, index: true

The t.reference part makes sense, but I don't know what index: true means. Can anyone tell me? I haven't been able to find that in the docs.

Note: This is not a duplicate of: Rails ActiveRecord::Migration what is the difference between index: true and add_index? Which only diffs the two, but doesn't explain what they do.

Answer 1

index: true adds a database index to the referenced column. For example, if creating a :products table:

create_table :products do |t|
  t.references :user, index: true
end

That will create a non-unique index on the user_id column in the products table named index_products_on_user_id .

Answer 2

well, when you create this add_reference thing you are saying to rails to add a user_id within "product table". i think the id's are useful to connect differents tables.