Question

For example, suppose there is the code in Rails 3.2.3

def test_action
  a = User.find_by_id(params[:user_id])
  # some calculations.....
  b = Reporst.find_by_name(params[:report_name])
  # some calculations.....
  c = Places.find_by_name(params[:place_name])
end

This code does 3 requests to database and opens 3 different connections. Most likely it’s going to be a quite long action.

Is there any way to open only one connection and do 3 requests within it? Or I want to control which connection to use by myself.

Answer 1

You would want to bracket the calls with transaction:

Transactions are protective blocks where SQL statements are only
permanent if they can all succeed as one atomic action. The classic
example is a transfer between two accounts where you can only have a
deposit if the withdrawal succeeded and vice versa. Transactions
enforce the integrity of the database and guard the data against
program errors or database break-downs. So basically you should use
transaction blocks whenever you have a number of statements that must
be executed together or not at all.

def test_action
  User.transaction do
    a = User.find_by_id(params[:user_id])
    # some calculations.....
    b = Reporst.find_by_name(params[:report_name])
    # some calculations.....
    c = Places.find_by_name(params[:place_name])
  end
end

Even though they invoke different models the actions are encapsulated into one call to the DB. It is all or nothing though. If one fails in the middle then the entire capsule fails.

Though the transaction class method is called on some Active Record
class, the objects within the transaction block need not all be
instances of that class. This is because transactions are per-database
connection, not per-model.