Activerecord update_by_case

This Rails gem adds two methods to the ActiveRecord::Base class that allow you to update many records on a single database hit, using a case sql statement for it.

• update_by_case!(cases)
• update_by_case(cases)

The only difference between both methods is that update_by_case! (with a bang) raises any exception and the other simply returns true or false depending if the update was successful or not.

Add to your Gemfile:

gem 'update_by_case'

Then run bundler

Pass a cases hash as the only one argument to update_by_case(cases) where cases must have the following structure:

• Any key on the first level will be an attribute of the model to be updated.
• Inside any first level's keys nest a new hash with one key that will be the comparison field for the cases.
• Any pair of key: value inside the comparison field hash will be the case and the value to update for each case.

Having a model PatientTemperature with attributes:

• id
• temperature
• passport_number
• flight_number

And data in patient_temperatures table:

Then we want to update all Patient's temperatures based on its passport_number on a single database hit, we build first the next cases hash:

cases = {
  temperature: {
    passport_number: {
      111 => 37.5,
      222 => 39.2,
      333 => 36.1,
    }
  }
}

And then on the model PatientTemperature we use update_by_case(cases) method as follow:

PatientTemperature.update_by_case(cases)

This will generate a SQL UPDATE like:

UPDATE patient_temperatures 
SET temperature = CASE 
  WHEN passport_number = 111 then 37.5
  WHEN passport_number = 222 then 39.2
  WHEN passport_number = 333 then 36.1
  ELSE temperature
  END

The result will be the update of all records on a single SQL query.

You can include a WHERE clause inside your cases Hash too, this way you limit the number of records to be processed on large tables:

cases = {
  temperature: {
    passport_number: {
      111 => 37.5,
      222 => 39.2,
      333 => 36.1,
    }
  },
  where: { flight_number: 755 }
}

Generating the same SQL but with a WHERE clause like normal ActiveRecord where method.

UPDATE patient_temperatures 
SET temperature = CASE 
  WHEN passport_number = 111 then 37.5
  WHEN passport_number = 222 then 39.2
  WHEN passport_number = 333 then 36.1
  ELSE temperature
  END
WHERE flight_number = 755

If you ommit the comparison field, this will be assumed to be the id field, for example:

cases = {
  temperature: {
    1 => 37.5,
    2 => 39.2,
    3 => 36.1,
  }
}

Will generate SQL as follows:

UPDATE patient_temperatures 
SET temperature = CASE
  WHEN id = 1 then 37.5
  WHEN id = 2 then 39.2
  WHEN id = 3 then 36.1
  ELSE temperature
  END

The resulting SQL statement is generated by Arel, so it makes sanitization over query parameters to prevent SQL injection.

On large tables, you should scope the amount of records to process including a where options on cases Hash, that normally will be the same subset that is being updated with the cases.

You can add more than one field to be updated, for example, updating temperature and passport_number for each instance of PatientTemperature based on they id should pass a Hash like the following:

cases = {
  temperature: {
    1 => 37.5,
    2 => 39.2,
    3 => 36.1,
  },
  passport_number: {
    1 => 111,
    2 => 222,
    3 => 333,
  },
  where: { id: [1, 2, 3] }
}

Will produce the following SQL:

UPDATE patient_temperatures 
SET temperature = CASE 
  WHEN id = 1 then 37.5
  WHEN id = 2 then 39.2
  WHEN id = 3 then 36.1
  ELSE temperature
  END,
passport_number = CASE
  WHEN id = 1 then 111
  WHEN id = 2 then 222
  WHEN id = 3 then 333
  ELSE passport_number
  END
WHERE id in (1, 2, 3)

Bug reports and pull requests are welcome!

The gem is available as open source under the terms of the MIT License.