TLDR
In Ruby on Rails, an association is a connection between two Active Record models. An association can be a connection of type one-to-one with another model (class) An association indicates that the declaring model can be matched with zero or more instances of another model by proceeding through a third model. Understanding associations is vital to create a web application that behaves as you expect and avoid a good dose of frustration. If the current class contains the foreign key must use "belongs_to" instead of "belong_to_many"via the TL;DR App
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