# Tips and tricks ## Tips and Tricks ### ActiveRecord #### **Generic warnings** ActiveRecord is a very powerful component of rails. That's an abstraction between our models and the database, which allows to achieve many things. But sometimes, that comes at the cost of the performance. Ruby is good for many things, but manipulating a lot of data in Ruby can be sometimes very inefficient due to the cost of the memory that we use. So, when using ActiveRecord, let's think about the produced SQL statements and think if we could not optimize ActiveRecord usage by offloading some part of the work to the database. For instance: ```ruby # Don't do Account.last(10).map(&:name) # It produces SELECT * FROM accounts ; Then it selects the name # Do Account.last(10).pluck(:name) # It produces SELECT name FROM accounts ; So it fetches less data # Don't do Account.last(10).group_by(&:country).map do |k, v| [k, v.count] end # It produces SELECT * FROM accounts ; Then it groups # Do Account.last(10).group(:country).count.to_a # It produces SELECT company, count(*) FROM accounts GROUP BY country # so, this lets the database making the work ``` #### N+1 request One big performance killer of rails is the N+1 request. \ N+1 can occur in scenarios as the following: ```ruby Account.last(10).each { |account| puts(account.admin.name) } ``` In that case, we can see in the database that the queries produced are: ``` SELECT * FROM admin WHERE account_id = xxx SELECT * FROM admin WHERE account_id = xxx SELECT * FROM admin WHERE account_id = xxx SELECT * FROM admin WHERE account_id = xxx SELECT * FROM admin WHERE account_id = xxx SELECT * FROM admin WHERE account_id = xxx SELECT * FROM admin WHERE account_id = xxx SELECT * FROM admin WHERE account_id = xxx ``` That's because we're asking the name of the account's admin, which is loaded, as with the account's admin while trying to access that value. Instead, it's better to pre-fetch all the data, using one of the following techniques: ```ruby # Method 1, using active record Account.include(:admin).last(10).each do ... end # or Account.join(:admin).last(10).each do ... end # Method 2, doing manual pre-fetching Admin.where(account_id: Account.last(10).pluck(:id)).each do ... end # Method 3, doing a hash admins = Admin.where(account_id: Account.last(10).pluck(:id)) .group_by(&:account_id) Account.last(10).each do |account| puts(admins[account.id].first.name) end ``` #### Reset columns information in a migration *r*[*eset\_column\_information*](https://api.rubyonrails.org/classes/ActiveRecord/ModelSchema/ClassMethods.html#method-i-reset_column_information) is a useful method when just after creating a table or a column in a migration, you want to populate it with some default values. Without it, the following migration will throw an error on line 7 stating that *is\_happy* does not exist on the table *employees*: ```ruby class AddHappyToEmployees < ActiveRecord::Migration[5.1] def change add_column :employees, :is_happy, :boolean, default: false Employee.reset_column_information Employee.where(company: 'Prospect.io').update_all(is_happy: true) end end ``` ### Rails Migration #### Interesting article on Zero Downtime Read this article to better understand steps you can put in place to ensure that you rails migration are not locking the DB in production. ### ### Rails console #### Using reload! When interacting into the rails console, using `reload!` will reflect changes made into the code immediately, without the need to quit and re-open console. ``` $ rails c pry> Account.new_method NoMethodError: undefined method `new_method' for # Did you mean? undef_method // define `new_method` into account.rb pry> reload! // `new_method` will immidiately be available pry> Account.new_method ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://prospect-io.gitbook.io/developer-playbook/our-stack/ruby-on-rails/tips-and-tricks.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.