gistfile1.rb

class PlantBucketAssignment
  belongs_to :plant
  belongs_to :bucket
end

class Bucket
  has_many :plant_bucket_assignments
  has_many :plants, through: :plant_bucket_assignments

  def current_plant
    plant.sort_by(:created_at).last
  end
end

class Plant
  has_many :plant_bucket_assignments
  has_many :buckets, through: :plant_bucket_assignments

  def current_bucket
    buckets.sort_by(:created_at).last
  end

  def current_bucket=(bucket)
    buckets << bucket
  end
end