ActiveRecord中的计数关联

Question

我有两个模型，User和Group，其中组包含许多用户。如果我想使用一个查询来统计每个组中的用户数，则可以使用以下SQL：

select id, (select count(1) from users where group_id = groups.id) from groups

使用ActiveRecord是否可以有效地做到这一点？

为清楚起见，此查询将列出所有组ID，以及每个组中的用户数。

Answer 1

使用关联

1. group = Group.find(1) #find group with id = 1
   group.users.count    # count users whose group_id = 1 calls db everytime
    or  
   group.users.size  # get size from cache if group.users loaded

或直接

2. User.where(:group_id=>1).count

count助手在指定条件下在数据库上触发一个count（*）查询检查更多选项http://apidock.com/rails/ActiveRecord/Calculations/count

我也建议您通过rails guides

Answer 2

Group.all(
  :select => "groups.id, count(u.group_id) as users_count",
  :joins  => "LEFT OUTER JOIN users u ON u.group_id = groups.id",
  :group  => "groups.id"
)

Answer 3

subquery = User.where("users.group_id = groups.id").select('count(1)')
groups_with_count = Group.select(:id, "(#{subquery.to_sql}) as users_count")

或对相同结果使用sql分组

groups_with_count = Group.joins(:users).select(:id, 'count(users.id) as users_count').group(:id)

在两种情况下，您现在都可以使用MINIMAL原始sql在一个查询中获得结果：

groups_with_count.each { |group| puts "#{group.id} => #{group.users_count}" }

附加说明
您可以使用以下帮助程序将第一个子查询编写为subquery = User.via(:group).select('count(1)')，这是更简单且可维护的imo。
我已经在多个项目中使用此代码，以便编写更好的子查询：
class ApplicationRecord < ActiveRecord::Base
    # transform Raw sql that references an association such as: Shift.where('shifts.id = checkins.shift_id')
    # into a simpler version Shift.via(:checkin) if shift have the checkin relationship
    # No support for polymorphic association
    # Basic support for "through" reflection (using join)
    def via(name)
        association = reflect_on_association(name)

        raise ArgumentError, "#{name} is not a valid association of #{self.class.name}" unless association
        raise NotImplementedError if association.polymorphic?

        join_keys = association.join_keys
        table_column = arel_table[join_keys.foreign_key]
        association_column = Arel::Table.new(association.table_name)[join_keys.key]

        if association.through_reflection?
            through_association = association.through_reflection
            table_column = Arel::Table.new(through_association.table_name)[join_keys.foreign_key]

            joins(through_association.name).where(table_column.eq(association_column))
        else
            where(table_column.eq(association_column))
        end
    end
end
```