Add linkedWorkItems field to EpicType

What does this MR do and why?

Related to #499772 (closed)

This MR adds the linkedWorkItems field to EpicType on GraphQL. This field exposes a list of work items that are related, blocking or blocked by the epic.

Because epics are a type of work item, we can reuse WorkItems::LinkedItemsResolver to return the items linked to the epic. This means that the same data is already available for WorkItemType but this change is still required for parts of the application that use the legacy endpoints.

Example queries

• No filter

query workItemsLinkedToEpic {
  group(fullPath: "flightjs") {
    epic(iid: "8") {
      linkedWorkItems {
        nodes {
          title
        }
      }
    }
  }
}

response

{
  "data": {
    "group": {
      "epic": {
        "linkedWorkItems": {
          "nodes": [
            {
              "title": "Blocking Task"
            },
            {
              "title": "Blocked Task"
            },
            {
              "title": "Related Task"
            }
          ]
        }
      }
    }
  },
  "correlationId": "01JH0D6SG7MW25TRR988FVNGRC"
}

• Returns work items related to the epic

query workItemsLinkedToEpic {
  group(fullPath: "flightjs") {
    epic(iid: "8") {
      linkedWorkItems(filter: RELATED) {
        nodes {
          title
        }
      }
    }
  }
}

response

{
  "data": {
    "group": {
      "epic": {
        "linkedWorkItems": {
          "nodes": [
            {
              "title": "Related Task"
            }
          ]
        }
      }
    }
  },
  "correlationId": "01JH0D87FJMXGYW8B88RY5X54S"
}

• Returns work items blocked by the epic

query workItemsLinkedToEpic {
  group(fullPath: "flightjs") {
    epic(iid: "8") {
      linkedWorkItems(filter: BLOCKS) {
        nodes {
          title
        }
      }
    }
  }
}

response

{
  "data": {
    "group": {
      "epic": {
        "linkedWorkItems": {
          "nodes": [
            {
              "title": "Blocked Task"
            }
          ]
        }
      }
    }
  },
  "correlationId": "01JH0DANKNZ0EVMYTQ1CN46RCK"
}

• Returns work items blocking the epic

query workItemsLinkedToEpic {
  group(fullPath: "flightjs") {
    epic(iid: "8") {
      linkedWorkItems(filter: BLOCKED_BY) {
        nodes {
          title
        }
      }
    }
  }
}

response

{
  "data": {
    "group": {
      "epic": {
        "linkedWorkItems": {
          "nodes": [
            {
              "title": "Blocking Task"
            }
          ]
        }
      }
    }
  },
  "correlationId": "01JH0DBZ95BRJWNZRJ8RKX7EWG"
}

References

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MR acceptance checklist

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Screenshots or screen recordings

No UX changes

How to set up and validate locally

1. In a rails console generate the data and the query

group, project, author = Group.first, Project.first, User.first
epic_work_item = FactoryBot.create(:epic, :with_synced_work_item, group: group, author: author).work_item

FactoryBot.create(:work_item, :task, title: 'related task', project: project).tap { |task| FactoryBot.create(:work_item_link, source: epic_work_item, target: task) }

FactoryBot.create(:work_item, :task, title: 'blocked task', project: project).tap { |task| FactoryBot.create(:work_item_link, source: epic_work_item, target: task, link_type: 'blocks') }

FactoryBot.create(:work_item, :task, title: 'blocking task', project: project).tap { |task| FactoryBot.create(:work_item_link, source: task, target: epic_work_item, link_type: 'blocks') }

query = <<~QUERY
  query workITemsLinkedToEpic {
    group(fullPath: "#{group.full_path}") {
      epic(iid: "#{epic_work_item.iid}") {
        linkedWorkItems {
          edges {
            node {
              title
            }
          }
        }
      }
    }
  }
QUERY

print(query)

2. Login with root user and verify the printed query in http://gdk.test:3000/-/graphql-explorer
3. Verify the filtering by link type works correctly by adding the filter argument with the values RELATED, BLOCKS and BLOCKED_BY (as shown in the example queries above)