How to get earliest and latest dates for associated objects

问题

I am developing an Rails 4 app and I got two models. One is called Project and the other is called Task.

Project has many Tasks. Task belongs to Project.

The task objects have starts_at and ends_at attributes (datetime). How can I get the earliest starts_at and latest ends_at of all the associated tasks that belongs to a specific project?

I tried @project.tasks.max(:starts_at) but it did not work (got error).

回答1:

Use maximum instead of max.

max is for array maximum: [1,2].max == 2

http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-maximum

回答2:

Adapted from [Lars Haugseth's solution] to find next birthdays.

model.rb

def latest_tasks
  todays_date = Date.today
  start_days_from_now = todays_date - starts_at
  ends_days_from_now = ends_at - todays_date
  if start_days_from_now < ends_days_from_now
    yearmmdd = starts_at.strftime('%Y%m%d')
  else
    yearmmdd = ends_at.strftime('%Y%m%d')
  end
  return Date.parse("#{yearmmdd}")
end

Controller.rb

@project.tasks.where('starts_at >= ? or ends_at >=? ', Date.today, Date.today).sort_by(&:latest_tasks)

回答3:

I know this is an old question but for anyone looking to accomplish this, you could do it this way:

• Get an array of all the starts_at and ends_at values from @project.tasks:

  starts_at_values = @project.tasks.pluck(:starts_at)

  ends_at_values = @project.tasks.pluck(:ends_at)

• Select the Task record whose starts_at value is the earliest:

  earliest_task = @project.tasks.select{|task| task.starts_at == starts_at_values.min}

• Select the Task record whose ends_at value is the latest:

  latest_task = @project.tasks.select{|task| task.starts_at == ends_at_values.max}

If you need to only get the earliest and latest times, you could just do:

earliest_time = starts_at_values.min

latest_time = ends_at_values.max

来源：https://stackoverflow.com/questions/28091778/how-to-get-earliest-and-latest-dates-for-associated-objects