我的模型Cleaners在联合表Assignments中有很多Occurrence。 number_of_cleaners模型有一个字段Occurrences。
如何使用Active Record（或SQL，Postgres）找到所有Cleaners，其中分配的occurrences.number_of_cleaners数量小于Occurrences中指定的数量？
此查询用于标识Cleaners，我们需要在其中找到更多Occurrence以分配给class Occurrence < ActiveRecord::Base
belongs_to :job
has_many :assignments
has_many :cleaners, through: :assignments
end
class Assignment < ActiveRecord::Base
belongs_to :cleaner
belongs_to :occurrence
end
。
Occurrence作为旁注，我们之前只是查询了Assignment的{{1}}，而不管occurrences.number_of_cleaners。查询看起来像这样：
# Select future occurrences which do not have an assignment (and therefore no cleaners) and select one per job ordering by booking time
# The subquery fetches the IDs of all these occurrences
# Then, it runs another query where it gets all the IDs from the subquery and orders the occurrences by booking time
# See http://stackoverflow.com/a/8708460/1076279 for more information on how to perform subqueryes
subquery = Occurrence.future.where.not(id: Assignment.select(:occurrence_id).uniq).select('distinct on (job_id) id').order('occurrences.job_id', 'booking_time')
@occurrences = Occurrence.includes(job: :customer).where("occurrences.id IN (#{subquery.to_sql})").where.not(bundle_first_id: nil).select{ |occurrence| @current_cleaner.unassigned_during?(occurrence.booking_time, occurrence.end_time) }
答案 0 :(得分：1)
您应该在模型上实现counter_cache，而不是加入表格并进行查询，因为它更有效，并且完全符合此目的。
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