a8n: Introduce Threads in Store (sourcegraph#5509)

* a8n: Add threads table migration

* a8n: Implement Store.{Create,Count,List}Thread(s)

* Update schema.md

* a8n: Implement filtering by Campaign in Store.ListThreads

* a8n: Implement filtering support in Store.CountThreads

cmd/frontend/db/schema.md

@@ -43,6 +43,8 @@ Foreign-key constraints:
     "campaigns_author_id_fkey" FOREIGN KEY (author_id) REFERENCES users(id) ON DELETE CASCADE DEFERRABLE
     "campaigns_namespace_org_id_fkey" FOREIGN KEY (namespace_org_id) REFERENCES orgs(id) ON DELETE CASCADE DEFERRABLE
     "campaigns_namespace_user_id_fkey" FOREIGN KEY (namespace_user_id) REFERENCES users(id) ON DELETE CASCADE DEFERRABLE
+Referenced by:
+    TABLE "threads" CONSTRAINT "threads_campaign_id_fkey" FOREIGN KEY (campaign_id) REFERENCES campaigns(id) ON DELETE CASCADE DEFERRABLE
 
 ```
 
@@ -468,6 +470,7 @@ Check constraints:
 Referenced by:
     TABLE "default_repos" CONSTRAINT "default_repos_repo_id_fkey" FOREIGN KEY (repo_id) REFERENCES repo(id)
     TABLE "discussion_threads_target_repo" CONSTRAINT "discussion_threads_target_repo_repo_id_fkey" FOREIGN KEY (repo_id) REFERENCES repo(id) ON DELETE CASCADE
+    TABLE "threads" CONSTRAINT "threads_repo_id_fkey" FOREIGN KEY (repo_id) REFERENCES repo(id) ON DELETE CASCADE DEFERRABLE
 
 ```
 
@@ -570,6 +573,26 @@ Foreign-key constraints:
 
 ```
 
+# Table "public.threads"
+```
+   Column    |           Type           |                      Modifiers                       
+-------------+--------------------------+------------------------------------------------------
+ id          | bigint                   | not null default nextval('threads_id_seq'::regclass)
+ campaign_id | integer                  | not null
+ repo_id     | integer                  | not null
+ created_at  | timestamp with time zone | not null default now()
+ updated_at  | timestamp with time zone | not null default now()
+ metadata    | jsonb                    | not null default '{}'::jsonb
+Indexes:
+    "threads_pkey" PRIMARY KEY, btree (id)
+Check constraints:
+    "threads_metadata_check" CHECK (jsonb_typeof(metadata) = 'object'::text)
+Foreign-key constraints:
+    "threads_campaign_id_fkey" FOREIGN KEY (campaign_id) REFERENCES campaigns(id) ON DELETE CASCADE DEFERRABLE
+    "threads_repo_id_fkey" FOREIGN KEY (repo_id) REFERENCES repo(id) ON DELETE CASCADE DEFERRABLE
+
+```
+
 # Table "public.user_emails"
 ```
       Column       |           Type           |       Modifiers        

migrations/1528395586_add_threads_table.down.sql

@@ -0,0 +1,6 @@
+BEGIN;
+
+DROP TABLE IF EXISTS threads;
+
+COMMIT;
+

migrations/1528395586_add_threads_table.up.sql

@@ -0,0 +1,14 @@
+BEGIN;
+
+CREATE TABLE IF NOT EXISTS threads (
+  id bigserial PRIMARY KEY,
+  campaign_id integer NOT NULL REFERENCES campaigns(id)
+    ON DELETE CASCADE DEFERRABLE INITIALLY IMMEDIATE,
+  repo_id integer NOT NULL REFERENCES repo(id)
+    ON DELETE CASCADE DEFERRABLE INITIALLY IMMEDIATE,
+  created_at timestamp with time zone NOT NULL DEFAULT now(),
+  updated_at timestamp with time zone NOT NULL DEFAULT now(),
+  metadata jsonb NOT NULL DEFAULT '{}' CHECK (jsonb_typeof(metadata) = 'object')
+);
+
+COMMIT;