Add `parent()` relationships for GitHub porcelain (#255)

Test plan: Unit tests were added.
(Note: I haven't tested the error case, when there are an invalid number
of parents. I think this is OK for now, but I'm disclosing this so
reviewers can easily take issue with it. I'm planning to re-organize the
test code to be by method rather than by wrapper type (so the wrappers
section doesn't keep being a kitchen sink) and will hopefully
put that test in.)

src/plugins/github/porcelain.js

@@ -266,6 +266,7 @@ export class Author extends GithubEntity<AuthorNodePayload> {
     assertEntityType(e, AUTHOR_NODE_TYPE);
     return (e: any);
   }
+
   login(): string {
     return this.node().payload.login;
   }
@@ -280,12 +281,15 @@ export class PullRequest extends Commentable<PullRequestNodePayload> {
     assertEntityType(e, PULL_REQUEST_NODE_TYPE);
     return (e: any);
   }
+
   number(): number {
     return this.node().payload.number;
   }
+
   title(): string {
     return this.node().payload.title;
   }
+
   reviews(): PullRequestReview[] {
     return this.graph
       .neighborhood(this.nodeAddress, {
@@ -294,6 +298,11 @@ export class PullRequest extends Commentable<PullRequestNodePayload> {
       })
       .map(({neighbor}) => new PullRequestReview(this.graph, neighbor));
   }
+
+  parent(): Repository {
+    return (_parent(this): any);
+  }
+
   mergeCommitHash(): ?string {
     const mergeEdge = this.graph
       .neighborhood(this.nodeAddress, {
@@ -320,12 +329,18 @@ export class Issue extends Commentable<IssueNodePayload> {
     assertEntityType(e, ISSUE_NODE_TYPE);
     return (e: any);
   }
+
   number(): number {
     return this.node().payload.number;
   }
+
   title(): string {
     return this.node().payload.title;
   }
+
+  parent(): Repository {
+    return (_parent(this): any);
+  }
 }
 
 export class Comment extends Post<CommentNodePayload> {
@@ -333,6 +348,10 @@ export class Comment extends Post<CommentNodePayload> {
     assertEntityType(e, COMMENT_NODE_TYPE);
     return (e: any);
   }
+
+  parent(): Issue | PullRequest {
+    return (_parent(this): any);
+  }
 }
 
 export class PullRequestReview extends Post<PullRequestReviewNodePayload> {
@@ -340,10 +359,15 @@ export class PullRequestReview extends Post<PullRequestReviewNodePayload> {
     assertEntityType(e, PULL_REQUEST_REVIEW_NODE_TYPE);
     return (e: any);
   }
+
   state(): PullRequestReviewState {
     return this.node().payload.state;
   }
 
+  parent(): PullRequest {
+    return (_parent(this): any);
+  }
+
   comments(): PullRequestReviewComment[] {
     return this.graph
       .neighborhood(this.nodeAddress, {
@@ -361,4 +385,18 @@ export class PullRequestReviewComment extends Post<
     assertEntityType(e, PULL_REQUEST_REVIEW_COMMENT_NODE_TYPE);
     return (e: any);
   }
+  parent(): PullRequestReview {
+    return (_parent(this): any);
+  }
+}
+
+function _parent(x: Entity): Entity {
+  const parents = x.graph.neighborhood(x.address(), {
+    edgeType: "CONTAINS",
+    direction: "IN",
+  });
+  if (parents.length !== 1) {
+    throw new Error(`Bad parent relationships for ${stringify(x.address())}`);
+  }
+  return asEntity(x.graph, parents[0].neighbor);
 }

src/plugins/github/porcelain.test.js

@@ -73,6 +73,7 @@ describe("GitHub porcelain", () => {
       expect(repo.name()).toBe("example-github");
       expect(repo).toEqual(asEntity(graph, repo.address()));
     });
+
     it("Issues", () => {
       const issue = issueOrPRByNumber(1);
       expect(issue.title()).toBe("An example issue.");
@@ -86,7 +87,9 @@ describe("GitHub porcelain", () => {
       expect(issue.address()).toEqual(issue.node().address);
       expect(issue.authors().map((x) => x.login())).toEqual(["decentralion"]);
       expect(issue).toEqual(asEntity(graph, issue.address()));
+      expect(issue.parent()).toEqual(repo);
     });
+
     describe("PullRequests", () => {
       it("Merged", () => {
         const pullRequest = PullRequest.from(issueOrPRByNumber(3));
@@ -103,6 +106,7 @@ describe("GitHub porcelain", () => {
           "0a223346b4e6dec0127b1e6aa892c4ee0424b66a"
         );
         expect(pullRequest).toEqual(asEntity(graph, pullRequest.address()));
+        expect(pullRequest.parent()).toEqual(repo);
       });
       it("Unmerged", () => {
         const pullRequest = PullRequest.from(issueOrPRByNumber(9));
@@ -117,6 +121,7 @@ describe("GitHub porcelain", () => {
       expect(reviews[0].state()).toBe("CHANGES_REQUESTED");
       expect(reviews[1].state()).toBe("APPROVED");
       expect(reviews[0]).toEqual(asEntity(graph, reviews[0].address()));
+      expect(reviews[0].parent()).toEqual(pr);
     });
 
     it("Pull Request Review Comments", () => {
@@ -126,6 +131,7 @@ describe("GitHub porcelain", () => {
       const comments = reviews[0].comments();
       expect(comments).toHaveLength(1);
       const comment = comments[0];
+      expect(comment.parent()).toEqual(reviews[0]);
       expect(comment.url()).toBe(
         "https://github.com/sourcecred/example-github/pull/5#discussion_r171460198"
       );
@@ -148,6 +154,7 @@ describe("GitHub porcelain", () => {
       expect(comment.address()).toEqual(comment.node().address);
       expect(comment.authors().map((x) => x.login())).toEqual(["decentralion"]);
       expect(comment).toEqual(asEntity(graph, comment.address()));
+      expect(comment.parent()).toEqual(issue);
     });
 
     it("Authors", () => {