Alternative for Model.all - Model.where(condition)

问题

My question is quiet simple: How to optimize the following query to not get performance issues once the database is growing:

def projects_not_participating
  @project = Project.all - Project.joins(:participants).where(participant: {id: current_user.id})
end

My model setup is this:

def Project
  has_many :assignments
  has_many :participants, through: :assignments
end

def Participant
  has_many :assignments
  has_many :projects, through: assignments
end

I tried using

Project.joins(:participant).where.not(participant: {id: current_user.id})

but that returned all projects. I found out that the query is returning all table entries in assignments where participant_id is not the current_user.id and not grouping the project entries befor.

回答1:

Actually, you're very close to a solution. The current problem is duplicating projects. To avoid this, you can use distinct:

Project.joins(:participants).where.not(participants: {id: current_user.id}).distinct

回答2:

I finally found a solution. I added a method to my Project class:

def self.not_participating(user)
  where.not(id: user.projects)
end

and use this in my User controller:

def find_projects
  @projects = Project.not_participating(current_user)
end

来源：https://stackoverflow.com/questions/57948673/alternative-for-model-all-model-wherecondition