has_and_belongs_to_many, avoiding dupes in the join table

by

Prevent duplicates in the view only (Lazy solution)

The following does not prevent writing duplicate relationships to the database, it only ensures find methods ignore duplicates.

In Rails 5:

has_and_belongs_to_many :tags, -> { distinct }

Note: Relation#uniq was depreciated in Rails 5 (commit)

In Rails 4

has_and_belongs_to_many :tags, -> { uniq }

Prevent duplicate data from being saved (best solution)

Option 1: Prevent duplicates from the controller:

post.tags << tag unless post.tags.include?(tag)

However, multiple users could attempt post.tags.include?(tag) at the same time, thus this is subject to race conditions. This is discussed here.

For robustness you can also add this to the Post model (post.rb)

def tag=(tag)
  tags << tag unless tags.include?(tag)
end

Option 2: Create a unique index

The most foolproof way of preventing duplicates is to have duplicate constraints at the database layer. This can be achieved by adding a unique index on the table itself.

rails g migration add_index_to_posts
# migration file
add_index :posts_tags, [:post_id, :tag_id], :unique => true
add_index :posts_tags, :tag_id

Once you have the unique index, attempting to add a duplicate record will raise an ActiveRecord::RecordNotUnique error. Handling this is out of the scope of this question. View this SO question. 

rescue_from ActiveRecord::RecordNotUnique, :with => :some_method

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