Rails 嵌套连接 - Rails 4

Question

我有3个模型。

class Saving
  has_many :contributions
end

class Contribution
  belongs_to :saving
  has_many :deposits
end

class Deposit
  belongs_to :contribution
end

现在我想查询所有的储蓄和贡献，并计算deposits.amount的总和。我也想要没有存款的记录。

如果我使用

Saving.joins(contributions: [:deposit])

这将给我所有有存款的储蓄。但我想加入贡献和 left_outer_joins 存款。

我怎样才能做到这一点？我正在使用 Rails 4. 所以我无法做到

Saving.left_outer_joins(contributions: [:deposit]).

Answer 1

在遗留应用程序中进行外部连接的最直接的方法是使用 SQL 字符串：

Saving.select(
        'savings.*',
        'SUM(deposits.amount) AS total_deposits'
      )
      .joins(:contributions)
      .joins(%q{
         LEFT OUTER JOINS depostits 
         ON depostits.contribution_id = contributions.id
      })

您也可以（误）使用 .includes，因为它执行外部联接：

class Saving
  has_many :contributions
  has_many :deposits, through: :contributions
end

Saving.includes(:deposits)
      .references(:deposits) # forces a single query
      .joins(:contributions) # optional - creates an additional inner join
      .select(
        'savings.*',
        'SUM(deposits.amount) AS total_deposits'
      )

你也可以使用 Arel:

module LegacyOuterJoiner
  # clunky hack for outer joins in legacy apps.
  def arel_outer_joins(association_name, fkey = nil)
     raise "This method is obsolete! Use left_joins!" if respond_to?(:left_joins)
     other_model = reflect_on_association(association_name).klass
     right = other_model.arel_table
     fkey ||= "#{other_model.model_name.singular}_id".intern
     arel_table.joins(right, Arel::Nodes::OuterJoin)
       .on(arel_table[fkey].eq(right[:id])) 
       .join_sources
  end
end

class Deposit
  extend LegacyOuterJoiner
end

Saving.select(
        Saving.arel_table[Arel.star],
        Deposit.arel_table[:amount].sum.as('total_deposits')
      )
      .joins(:contributions)
      .joins(Deposit.arel_outer_joins(:contribution))

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