seeders.rb

module Seeders
  class Base
    def seed!(builder, findable_attributes, unstable_attributes = {})
      builder.find_or_create_by!(findable_attributes) do |config|
        config.assign_attributes(unstable_attributes)
      end
    end
  end

  class Builder
    def method_missing(name, *params, &block)
      matches = name.to_s.match(/(?<model>\w+)(?<bang>!)?(?<question>\?)?/)
      model, bang, _question = matches.values_at(:model, :bang, :question)
      plural = model.pluralize

      if name == plural
        name.classify.constantize
      elsif bang.present?
        if respond_to?(model)
          send(model, *params).tap(&:save!)
        else
          send(plural).create!(params, &block)
        end
      else
        send(plural).new(params, &block)
      end
    end

    def respond_to_missing?(name, *)
      name.classify.constantize
    rescue NameError
      false
    end
  end
end

usage.rb

builder = Builder.new

builder.widget # initializes a widget in memory
builder.widget! # creates a widget and saves it in the db

# …

class Builder < Seeders::Builder
    def widgets
      Widget.create_with(
        name: "A Widget",
        scrobbler: scrobbler
      )
    end

    def scrobbler
      Scrobbler.create_with(
        description: "A Scrobbler"
      )
    end

    def users
      User.create_with(
        first_name: "Bob",
        last_name: "Smith",
        email: "[email protected]",
        password: "password"
      )
    end
  end