tab/app/controllers/concerns/transactions_query.rb

class TransactionsQuery
  attr_reader :arel_table

  def initialize user
    @user = user
    @transactions = Arel::Table.new(:transactions)
    @perspectived = Arel::Table.new(:perspectived_transactions)
    @peers = Arel::Table.new(:users).alias('peers')
    @arel_table = Arel::Table.new("#{@user.username}_transactions")
  end

  def query
    Arel::SelectManager.new(ActiveRecord::Base)
      .from(arel)
  end

  def arel
    Arel::Nodes::TableAlias.new(
      issued_by(User),
      arel_table.name
    )
  end

  def issued_by klass
    issuers = klass.arel_table.alias('issuer')
    Arel::SelectManager.new(ActiveRecord::Base)
      .from(transactions)
      .join(@peers).on(@peers[:id].eq(@perspectived[:peer_id]))
      .join(issuers).on(issuers[:id].eq(@perspectived[:issuer_id]))
      .where(@perspectived[:issuer_type].eq(klass.name))
      .project(
        @perspectived[:amount],
        @perspectived[:time],
        @perspectived[:message],
        @peers[:name].as('peer'),
        issuers[:name].as('issuer')
      )
  end

  def transactions
    Arel::Nodes::TableAlias.new(
      incoming.union(:all, outgoing),
      @perspectived.name
    )
  end

  def outgoing
    @transactions
      .where(@transactions[:debtor_id].eq(@user.id))
      .project(
        (@transactions[:amount]*Arel::Nodes::SqlLiteral.new("-1")).as('amount'),
        @transactions[:creditor_id].as('peer_id'),
        @transactions[:created_at].as('time'),
        @transactions[:issuer_id],
        @transactions[:issuer_type],
        @transactions[:message]
      )
  end

  def incoming
    @user.incoming_transactions.arel
    @transactions
      .where(@transactions[:creditor_id].eq(@user.id))
      .project(
        @transactions[:amount],
        @transactions[:debtor_id].as('peer_id'),
        @transactions[:created_at].as('time'),
        @transactions[:issuer_id],
        @transactions[:issuer_type],
        @transactions[:message]
      )
  end
end