When we developed the gaming App for our client Colors, we encountered a common data consistency problem: How to prevent duplicated database record creation. Of course everyone instantly can think of "use #find_or_create_by", but it is actually little bit more complicated than that.


We are using Ruby on Rails to implement the backend. There is a ActiveRecord method #find_or_create_by to do the find or create:

Product.find_or_create_by(key: key)

the method signature should be easy to understand what it is trying to do: either get an existing record with the provided key, or create a new record with the provided key.

Issue 1

First and foremost, if you are not aware, this method is NOT atomic (surprise!):

from https://apidock.com/rails/v4.2.7/ActiveRecord/Relation/find_or_create_by

Please note *this method is not atomic*, it runs first a SELECT, and if there are no results an INSERT is attempted. If there are other threads or processes there is a race condition between both calls and it could be the case that you end up with two similar records.

If you concurrently run the #find_or_create_by in two separate processes, it is possible that two identical records will be created.

Issue 1 resolution

So, a unique constraint is required at the database level to prevent . The following migration step can solve that:

add_index :products, :key, :string, unique: true

What would happen when there are two concurrent #find_or_create_by methods executing at the same time ? a ActiveRecord::RecordNotUnique exception is thrown. Hence, the correct way should be

  Product.find_or_create_by(key: key)
rescue ActiveRecord::RecordNotUnique

Issue 2

There is another business requirement on DailyBonusLedger: maximum only one ledger per user per day should be created. Here is the simplified version of DailyBonusLedger schema:

user_id integer
created_at datetime
bonus_amount integer

Obviously, a compound unique constraint is needed across User ID and Ledger Date.

However, we cannot use the [:user_id, :created_at] columns as the compound unique index. Since the ledger is unique within a day, not within timestamp millisecond. And there is no way to generate a unique constraint on the "day" part of the :created_at column.

How can we achieve that with unique constraint ?

Issue 2 resolution

What we can do instead is to add an extra column (e.g. :ledger_date) to store the day part of current date. And then create a unique constraint on [:user_id, :ledger_date]:

add_index :daily_bonus_ledgers, [:user_id, :ledger_date], unique: true

When we create a new record, we extract the day part of current time using #beginning_of_day:

  DailyBonusLedger.find_or_create_by(user_id: user_id, ledger_date: Time.now.beginning_of_day)
rescue ActiveRecord::RecordNotUnique

With the new column and the compound unique constraint in place, only maximum one ledger is created per day per user.

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