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The problem occurs when we have to make a query to get each of the associations, "1" being the first query and "N" the number of queries required to get all the associations. Preload, Includes and eager_load are three ways to avoid this problem. The problem is because the queries are not handled in a proper manner this can slow down the loading, and that is because of the "N + 1" problem. If we use Preload we can't use a "where" clause in the associated table and with Includes we can fix the problem by making only two queries.