1 files changed, 119 insertions, 0 deletions
diff --git a/app/models/design_management/design_at_version.rb b/app/models/design_management/design_at_version.rb
new file mode 100644
index 00000000000..b4cafb93c2c
--- /dev/null
+++ b/app/models/design_management/design_at_version.rb
@@ -0,0 +1,119 @@
+# frozen_string_literal: true
+
+# Tuple of design and version
+# * has a composite ID, with lazy_find
+module DesignManagement
+  class DesignAtVersion
+    include ActiveModel::Validations
+    include GlobalID::Identification
+    include Gitlab::Utils::StrongMemoize
+
+    attr_reader :version
+    attr_reader :design
+
+    validates :version, presence: true
+    validates :design, presence: true
+
+    validate :design_and_version_belong_to_the_same_issue
+    validate :design_and_version_have_issue_id
+
+    def initialize(design: nil, version: nil)
+      @design, @version = design, version
+    end
+
+    def self.instantiate(attrs)
+      new(attrs).tap { |obj| obj.validate! }
+    end
+
+    # The ID, needed by GraphQL types and as part of the Lazy-fetch
+    # protocol, includes information about both the design and the version.
+    #
+    # The particular format is not interesting, and should be treated as opaque
+    # by all callers.
+    def id
+      "#{design.id}.#{version.id}"
+    end
+
+    def ==(other)
+      return false unless other && self.class == other.class
+
+      other.id == id
+    end
+
+    alias_method :eql?, :==
+
+    def self.lazy_find(id)
+      BatchLoader.for(id).batch do |ids, callback|
+        find(ids).each do |record|
+          callback.call(record.id, record)
+        end
+      end
+    end
+
+    def self.find(ids)
+      pairs = ids.map { |id| id.split('.').map(&:to_i) }
+
+      design_ids = pairs.map(&:first).uniq
+      version_ids = pairs.map(&:second).uniq
+
+      designs = ::DesignManagement::Design
+        .where(id: design_ids)
+        .index_by(&:id)
+
+      versions = ::DesignManagement::Version
+        .where(id: version_ids)
+        .index_by(&:id)
+
+      pairs.map do |(design_id, version_id)|
+        design = designs[design_id]
+        version = versions[version_id]
+
+        obj = new(design: design, version: version)
+
+        obj if obj.valid?
+      end.compact
+    end
+
+    def status
+      if not_created_yet?
+        :not_created_yet
+      elsif deleted?
+        :deleted
+      else
+        :current
+      end
+    end
+
+    def deleted?
+      action&.deletion?
+    end
+
+    def not_created_yet?
+      action.nil?
+    end
+
+    private
+
+    def action
+      strong_memoize(:most_recent_action) do
+        ::DesignManagement::Action
+          .most_recent.up_to_version(version)
+          .find_by(design: design)
+      end
+    end
+
+    def design_and_version_belong_to_the_same_issue
+      id_a, id_b = [design, version].map { |obj| obj&.issue_id }
+
+      return if id_a == id_b
+
+      errors.add(:issue, 'must be the same on design and version')
+    end
+
+    def design_and_version_have_issue_id
+      return if [design, version].all? { |obj| obj.try(:issue_id).present? }
+
+      errors.add(:issue, 'must be present on both design and version')
+    end
+  end
+end