Question

What I am trying to do is to come up with the model for Project, where only one project could be active at a time. So I am thinking to create one table for projects and another one for status.

How can I make sure that only one project is active at a given time ? I looked into validates_uniqueness_of but I am little confused if thats the solution for me, because I do want to able to override the current status, despite the fact if one project is already enabled.

Answer 1

Your Project should have something like an :active attribute. You can then add validation like this on your Project model

validate :ensure_only_one_active_project

def ensure_only_one_active_project
  active_project = Project.where(active: true).first

  errors.add(:active, "Another project is already active. Only one project can be active at a time") unless active_project.nil? || active_project.id == self.id
end

Now, if a Project already is active, and have another (currently inactive) Project @project, and try to do

@project.active = true
@project.valid? # this will return false

@project.errors will contain a Hash with :active attribute having the above error since

• 1 Project is already active
• The active project is not the current Project we’re saving.