Database-independent SQL String Concatenation in Rails

Question

I want to do a database-side string concatenation in a Rails query, and do it in database-independent way.

SQL-92 specifies double-bar (||) as the conca

Answer 1

It hasn't received much usage yet but I wrote the following code which seems to solve the problem. This monkey-patches the adapters to have a method to support it:

module ActiveRecord
  module ConnectionAdapters
    class AbstractAdapter

      # Will return the given strings as a SQL concationation. By default
      # uses the SQL-92 syntax:
      #
      #   concat('foo', 'bar') -> "foo || bar"
      def concat(*args)
        args * " || "
      end

    end

    class AbstractMysqlAdapter < AbstractAdapter

      # Will return the given strings as a SQL concationation.
      # Uses MySQL format:
      #
      #   concat('foo', 'bar')  -> "CONCAT(foo, bar)"
      def concat(*args)
        "CONCAT(#{args * ', '})"
      end

    end

    class SQLServerAdapter < AbstractAdapter

      # Will return the given strings as a SQL concationation.
      # Uses MS-SQL format:
      #
      #   concat('foo', 'bar')  -> foo + bar
      def concat(*args)
        args * ' + '
      end

    end
  end
end

With this you should be able to do the following in your code:

class User < ActiveRecord::Base

  def self.find_by_name(name)
    where("#{connection.concat('first_name', 'last_name')} = ?", name)
  end

end

This outputs the following SQL query on a SQL-92 database (Oracle, SQLite, PostgreSQL):

SELECT * FROM users WHERE first_name || last_name = ?

For MySQL it outputs:

SELECT * FROM users WHERE CONCAT(first_name, last_name) = ?

For SQL Server it outputs

SELECT * FROM users WHERE first_name + last_name = ?

Obviously you could extend this concept to other database adapters.