Optimize for use-case of only thread count

This is pre-mature optimization but it's a query that
is called on every page load for our main detail pages.
That's a hot path, so I'm taking the effort here.

src/components/comments/comment-thread.repository.ts

@@ -98,4 +98,18 @@ export class CommentThreadRepository extends DtoRepository(CommentThread) {
       .first();
     return result!; // result from paginate() will always have 1 row.
   }
+
+  async count(parent: ID) {
+    const result = await this.db
+      .query()
+      .match([
+        node('node', 'CommentThread'),
+        relation('in', '', 'commentThread', ACTIVE),
+        node('', 'BaseNode', { id: parent }),
+      ])
+      .return<{ count: number }>('count(node) as count')
+      .map('count')
+      .first();
+    return result!;
+  }
 }

src/components/comments/comment.service.ts

@@ -153,6 +153,18 @@ export class CommentService {
     }
   }
 
+  async getThreadCount(parent: Commentable, session: Session) {
+    const perms = await this.getPermissionsFromResource(parent, session);
+
+    // Do check here since we don't filter in the db query.
+    // Will need to be updated with DB switch.
+    if (!perms.can('read')) {
+      return 0;
+    }
+
+    return await this.repo.threads.count(parent.id);
+  }
+
   async listThreads(
     parent: Commentable,
     input: CommentThreadListInput,

src/components/comments/commentable.resolver.ts

@@ -1,7 +1,9 @@
-import { Parent, Query, ResolveField, Resolver } from '@nestjs/graphql';
+import { Info, Parent, Query, ResolveField, Resolver } from '@nestjs/graphql';
 import {
+  Fields,
   ID,
   IdArg,
+  IsOnly,
   ListArg,
   LoggedInSession,
   Resource,
@@ -31,7 +33,12 @@ export class CommentableResolver {
     @ListArg(CommentThreadListInput) input: CommentThreadListInput,
     @LoggedInSession() session: Session,
     @Loader(CommentThreadLoader) commentThreads: LoaderOf<CommentThreadLoader>,
-  ): Promise<CommentThreadList> {
+    @Info(Fields, IsOnly(['total'])) onlyTotal: boolean,
+  ) {
+    if (onlyTotal) {
+      const total = await this.service.getThreadCount(parent, session);
+      return { total };
+    }
     const list = await this.service.listThreads(parent, input, session);
     commentThreads.primeAll(list.items);
     return list;