Epics should preserve parent-child association during migration

Problem to Solve

With the implementation of the Epic migration MVC, the following Epic attributes are already being migrated:

• title
• description
• state
• createdAt
• closedAt
• startDate
• startDateFixed
• startDateIsFixed
• dueDateFixed
• dueDateIsFixed
• relativePosition
• confidential

This provides the basic info for each Epic, but leaves out any relations, such as Parent Epics, Labels, Comments, etc., which is something that users would expect is included in a full Group Migration.

For the users to confidently use the Group Migration feature, we need to iteratively deliver the import of the remaining Epic data, per this proposal.

Proposed Solution

1. Embed the parent epic on the Epic Query

fragment EpicFields on Epic {
  title
  description
  state
  create_at: createdAt
  closed_at: closedAt
  start_date: startDate
  start_date_fixed: startDateFixed
  start_date_is_fixed: startDateIsFixed
  due_date_fixed: dueDateFixed
  due_date_is_fixed: dueDateIsFixed
  relative_position: relativePosition
  confidential
}

query ($full_path: ID!, $cursor: String) {
  group(fullPath: $full_path) {
    epics(includeDescendantGroups: false, first: 100, after: $cursor) {
      pageInfo {
        endCursor
        hasNextPage
      }
      nodes {
        ...EpicFields
        parent {
          ...EpicFields
        }
      }
    }
  }
}

2. Add a new EpicParentTransformer.
   • If hasParent -> try to locate it using parent's title/description/createdAt.
   • If it's located - replace 'parent' key with epic object.
   • If not - build a new epic within 'parent' key.
3. During saving we might be potentially creating 2 epics, a child and parent at the same time.

Documentation

• User docs: https://docs.gitlab.com/ee/user/group/import/ (WIP)
• Dev docs: https://docs.gitlab.com/ee/development/bulk_import.html

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