Rails 3.2 query using "WHERE IN" doesn't use my primary key as an index
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I have a Rails 3.2 app where I am trying to grab a group of users. My Rails code for the query looks like:

    @users = User.where("user_id IN (?)", foo)

which generates the following SQL:

    SELECT 'users'.* FROM 'users' WHERE id IN (1, 2, 3, 4)

When I run an explain on this query, it is searching all rows in my users table. It lists <code>PRIMARY</code> under the <code>possible_keys</code> field, but the key is showing as null.

If I remove the "IN (?)" part and pull just one record:

    @user = User.where("user_id = ?", bar)

generating

    SELECT 'users'.* FROM 'users' WHERE id = 1

then the query is executed using the primary key as the index and searching only a single row.

Any idea why this isn't working in my first example? Am I structuring this query incorrectly in Rails?