diff --git a/src/categories/unread.ts b/src/categories/unread.ts
new file mode 100644
index 00000000..cb053289
--- /dev/null
+++ b/src/categories/unread.ts
@@ -0,0 +1,43 @@
+import db from '../database';
+
+interface CategoriesModel {
+  markAsRead(cids: number[], uid: number): Promise<void>;
+  markAsUnreadForAll(cid: number): Promise<void>;
+  hasReadCategories(cids: number[], uid: number): Promise<boolean[]>;
+  hasReadCategory(cid: number, uid: number): Promise<boolean>;
+}
+
+export default function setupCategoriesModel(Categories: CategoriesModel): void {
+  Categories.markAsRead = async function (cids: number[], uid: number): Promise<void> {
+    if (!Array.isArray(cids) || !cids.length || uid <= 0) {
+      return;
+    }
+    let keys = cids.map(cid => `cid:${cid}:read_by_uid`);
+    const hasRead = await db.isMemberOfSets(keys, uid);
+    keys = keys.filter((key, index) => !hasRead[index]);
+    await db.setsAdd(keys, uid);
+  };
+
+  Categories.markAsUnreadForAll = async function (cid: number): Promise<void> {
+    if (!cid) {
+      return;
+    }
+    await db.delete(`cid:${cid}:read_by_uid`);
+  };
+
+  Categories.hasReadCategories = async function (cids: number[], uid: number): Promise<boolean[]> {
+    if (uid <= 0) {
+      return cids.map(() => false);
+    }
+
+    const sets = cids.map(cid => `cid:${cid}:read_by_uid`);
+    return await db.isMemberOfSets(sets, uid);
+  };
+
+  Categories.hasReadCategory = async function (cid: number, uid: number): Promise<boolean> {
+    if (uid <= 0) {
+      return false;
+    }
+    return await db.isSetMember(`cid:${cid}:read_by_uid`, uid);
+  };
+}