Add disable_joins option to has_one relation by cheshire137 · Pull Request #4207...
source link: https://github.com/rails/rails/pull/42079
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Summary
This adds disable_joins as an option for has_one ... through: relations, like what #41937 did for has_many ... through:. The goal is to be able to define has_one relationships whose tables are in separate database clusters in a multi-database Rails app. Joins need to be avoided in this case because you can't join across clusters.
Other Information
I've got some failing tests when I run has_one_through_disable_joins_associations_test.rb locally, and I'm not sure if the failures are due to an implementation problem I need to fix or if the test is wrong. 
My approach for adding tests was to copy tests from activerecord/test/cases/associations/has_one_through_associations_test.rb and duplicate any existing has_one ... through: relations to add a *_without_joins version using disable_joins: true.
cc @eileencodes who encouraged me and was so helpful adding initial disable_joins support 
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