Handling unique constraint exceptions inside transaction - ruby-on-rails

Question

I have two Rails models / tables that I want insert as part of transaction.
For example, here are roughly my tables:
Posts: (id)
Comments: (id, post_id, comment_text, user_id)
Commenters: (id, post_id, user_id), unique constraint on (post_id,
user_id)
Right now I'm trying approximately equivalent to:
ActiveRecord::Base.transaction do
Comment.create!(post: post, user: user, comment_text: '...')
begin
Commenters.find_or_create_by!(post: post, user: user)
rescue PG::UniqueViolation
end
end
This works 99.9% of time, but sometimes two concurrent comments will trigger a PG::UniqueViolation.
Even though I'm catching and suppressing the PG::UniqueViolation, the entire transaction fails due to:
ERROR: current transaction is aborted, commands ignored until end of transaction block
I realize I could already achieve this by joining Post and Comment table, but this is a simplified example.
Is there a simpler way to ensure both inserts happen as part of a transaction while still ignoring the unique violation since we can assume that the record already exists?

Answer 1

An exception raised inside the transaction does two things:
Rolls back the transaction so none of the changes made inside the transaction will persist in the database.
The exception propagates outside the transaction block.
So you can move your exception handler outside the transaction call:
begin
ActiveRecord::Base.transaction do
Comment.create!(post: post, user: user, comment_text: '...')
Commenters.find_or_create_by!(post: post, user: user)
end
rescue PG::UniqueViolation
retry
end
You could include a counter to only retry a few times if you wanted more safety.

Answer 2

You should have the associations properly set inside your models, so rails does the validation for you. Then you can simply rescue the possible ActiveRecord::RecordInvalid (with message Validation failed: Attribute has already been taken).
If you want to read some more about uniqueness validtion this should come in handy: http://guides.rubyonrails.org/active_record_validations.html#uniqueness
To me it actually looks like your Commenter model is not really necessary. It just consists of derived information which can also be drawn directly from the Comments model (there you already store post_id and user_id) so you could drop the Commenter class entirely. Then take care to validate Comment on creation by for example setting
class Comment
belongs_to :post
belongs_to :user
validates :user_id, uniqueness: {scope: :post_id}
end
But this way you would allow a user to comment only once.
I propose you drop the uniqueness constraint and construct the information stored in Commenter by making distinct selects on the Comment model.
PS: Models in rails are written in Uppercase and singular while tables are referred to (by symbols) in lowercase and plural.

Answer 3

The error itself happens because of multi-threading behaviour of your app.
You need to rescue ActiveRecord::RecordNotUnique instead of PG-specific error.
Also perhaps put transaction inside begin rescue end block.
And retry to continue with another transaction within rescue block. Something like another answer suggested.