class Bucket < ApplicationRecord
  validates :scenario, presence: true, if: -> (bucket) { false }
end

irb(main):001:0> b = Bucket.new(name: 'test', size: 0, bucket_preset: BucketPreset.first, default_transaction_type: 0)
  BucketPreset Load (0.3ms)  SELECT  "bucket_presets".* FROM "bucket_presets" ORDER BY "bucket_presets"."id" ASC LIMIT $1  [["LIMIT", 1]]
=> #<Bucket id: nil, name: "test", scenario_id: nil, size: 0, created_at: nil, updated_at: nil, bucket_preset_id: 1, default_transaction_type: "expense">

irb(main):002:0> b.valid?
=> false

irb(main):004:0> b.errors.first
=> [:scenario, "must exist"]