Rails 5 Warning when fetching with Active Record

With large data set we can run into memory issue. Here is an example.

1
2>> Post.published.count
3=> 25000
4
5>> Post.where(published: true).each do |post|
6     post.archive!
7   end
8
9# Loads 25000 posts in memory
10
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Rails 5 adds warning when loading large data set

To mitigate issue shown above Rails 5 has added config.active_record.warn_on_records_fetched_greater_than.

When this configuration is set to an integer value, any query that returns the number of records greater than the set limit, logs a warning.

1
2config.active_record.warn_on_records_fetched_greater_than = 1500
3
4>> Post.where(published: true).each do |post|
5     post.archive!
6   end
7
8=> Query fetched 25000 Post records: SELECT "posts".* FROM "posts" WHERE "posts"."published" = ? [["published", true]]
9   [#<Post id: 1, title: 'Rails', user_id: 1, created_at: "2016-02-11 11:32:32", updated_at: "2016-02-11 11:32:32", published: true>, #<Post id: 2, title: 'Ruby', user_id: 2, created_at: "2016-02-11 11:36:05", updated_at: "2016-02-11 11:36:05", published: true>,....]
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This helps us find areas where potential problems exist and then we can replace inefficient queries with better ones.

1
2config.active_record.warn_on_records_fetched_greater_than = 1500
3
4>> Post.where(published: true).find_each do |post|
5     post.archive!
6   end
7
8# No warning is logged
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