When using redirect_to or render twice in one action, we get the AbstractController::DoubleRenderError error.

For example, I had the following action with two redirect_to methods:

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def create
  if @user.has_statement_this_month?(@account)
    flash[:alert] = "You already have a statement for this month."
    redirect_to(account_path(@account))
  end

  @statement = Statement.new(statement_params)
  @statement.account_id = @account.id

  if @statement.save
    flash[:notice] = 'Statement created'
    redirect_to(account_path(@account))
  else
    flash[:alert] = @statement.errors.full_messages.join(', ')
    render('new')
  end
end

In this case, if a statement has an existing statement record for the current month, the action should redirect the user to account_path. This part works. However, redirects do not stop execution of the remaining code. This means that in the code above, it is possible to try to redirect to account_path as well as render the new template. This results in an error: AbstractController::DoubleRenderError.

To resolve this we need to add a return statement after the redirect_to in order to exit out of the action:

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if @user.has_statement_this_month?(@account)
  flash[:alert] = "You already have a statement for this month."
  redirect_to(account_path(@account))
  return # add this
end

Sometimes I forget that actions are just regular ruby methods and can be exited out of by using return.

More information is available here: Avoiding Double Render Errors