01 table

    create_table :monthly_transactions do |t|
          t.integer :jaar
      t.integer :maand
      t.integer :cijfer_1
      t.decimal :saldo

02 model

class MonthlyTransaction < ActiveRecord::Base
  scope :saldo_per_month, -> { select( [ :jaar, :maand, MonthlyTransaction.arel_table[:saldo].sum ] )
                                .where( 
                                  Arel::Nodes::Group.new(
                                    MonthlyTransaction.arel_table[:cijfer_1].eq('6').or(MonthlyTransaction.arel_table[:cijfer_1].eq('7'))
                                  )
                                ).group(:jaar, :maand) }
end

03 error

MonthlyTransaction.saldo_per_month[1].saldo
  MonthlyTransaction Load (4.9ms)  SELECT "monthly_transactions"."jaar", "monthly_transactions"."maand", SUM("monthly_transactions"."saldo") FROM "monthly_transactions" WHERE (("monthly_transactions"."cijfer_1" = 6 OR "monthly_transactions"."cijfer_1" = 7)) GROUP BY "monthly_transactions"."jaar", "monthly_transactions"."maand"
ActiveModel::MissingAttributeError: missing attribute: saldo        from /usr/local/rvm/gems/ruby-2.2.1/gems/activerecord-4.2.4/lib/active_record/attribute_methods/read.rb:93:in `block in _read_attribute'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activerecord-4.2.4/lib/active_record/attribute_set.rb:31:in `block in fetch_value'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activerecord-4.2.4/lib/active_record/attribute.rb:150:in `value'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activerecord-4.2.4/lib/active_record/attribute_set.rb:31:in `fetch_value'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activerecord-4.2.4/lib/active_record/attribute_methods/read.rb:93:in `_read_attribute'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activerecord-4.2.4/lib/active_record/attribute_methods.rb:50:in `__temp__3716c646f6'
        from (irb):1
        from /usr/local/rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands/console.rb:110:in `start'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands/console.rb:9:in `start'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands/commands_tasks.rb:68:in `console'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands.rb:17:in `<top (required)>'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:274:in `require'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:274:in `block in require'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:240:in `load_dependency'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:274:in `require'
        from /home/ubuntu/workspace/bin/rails:8:in `<top (required)>'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:268:in `load'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:268:in `block in load'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:240:in `load_dependency'
        from /usr/local/rvm/gems/ruby-2.2.1/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:268:in `load'
        from /usr/local/rvm/rubies/ruby-2.2.1/lib/ruby/site_ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /usr/local/rvm/rubies/ruby-2.2.1/lib/ruby/site_ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'