diff --git a/ghost/collections/src/Collection.ts b/ghost/collections/src/Collection.ts
index baa29848cd..ba44f9dd7e 100644
--- a/ghost/collections/src/Collection.ts
+++ b/ghost/collections/src/Collection.ts
@@ -1,10 +1,8 @@
-// have to use requires until there are type definitions for these modules
-
 import {ValidationError} from '@tryghost/errors';
 import tpl from '@tryghost/tpl';
 
 import ObjectID from 'bson-objectid';
-import {PostDTO} from './PostDTO';
+import {CollectionPost} from './CollectionPost';
 
 const messages = {
     invalidIDProvided: 'Invalid ID provided for Collection',
@@ -23,7 +21,7 @@ export class Collection {
     updatedAt: Date;
     deleted: boolean;
 
-    posts: PostDTO[];
+    posts: CollectionPost[];
 
     private constructor(data: any) {
         this.id = data.id;
@@ -52,12 +50,26 @@ export class Collection {
             updatedAt: this.updatedAt,
             posts: this.posts.map(post => ({
                 id: post.id,
-                title: post.title,
-                slug: post.slug
+                postId: post.postId,
+                sortOrder: post.sortOrder
             }))
         };
     }
 
+    async canAddPost(newCollectionPost: CollectionPost): Promise<Boolean> {
+        const existingCollectionPost = this.posts.find(p => p.id === newCollectionPost.id);
+
+        if (existingCollectionPost) {
+            return false;
+        }
+
+        return true;
+    }
+
+    addPost(collectionPost: CollectionPost) {
+        this.posts.push(collectionPost);
+    }
+
     static validateDateField(date: any, fieldName: string): Date {
         if (!date) {
             return new Date();
diff --git a/ghost/collections/src/CollectionPost.ts b/ghost/collections/src/CollectionPost.ts
new file mode 100644
index 0000000000..9b89e69316
--- /dev/null
+++ b/ghost/collections/src/CollectionPost.ts
@@ -0,0 +1,47 @@
+import {ValidationError} from '@tryghost/errors';
+import tpl from '@tryghost/tpl';
+import ObjectID from 'bson-objectid';
+
+const messages = {
+    invalidIDProvided: 'Invalid ID provided for CollectionPost'
+};
+
+export class CollectionPost {
+    id: string;
+    postId: string;
+    collectionId: string;
+    sortOrder: number;
+    deleted: boolean;
+
+    constructor(data: any) {
+        this.id = data.id;
+        this.postId = data.postId;
+        this.collectionId = data.collectionId;
+        this.sortOrder = data.sortOder;
+        this.deleted = data.deleted;
+    }
+
+    static async create(data: any): Promise<CollectionPost> {
+        let id;
+
+        if (!data.id) {
+            id = new ObjectID();
+        } else if (typeof data.id === 'string') {
+            id = ObjectID.createFromHexString(data.id);
+        } else if (data.id instanceof ObjectID) {
+            id = data.id;
+        } else {
+            throw new ValidationError({
+                message: tpl(messages.invalidIDProvided)
+            });
+        }
+
+        return new CollectionPost({
+            id: id.toHexString(),
+            postId: data.post_id,
+            collectionId: data.collection_id,
+            sortOrder: data.sort_order, // NOTE: make sort_order required during creation
+            deleted: false
+        });
+    }
+}
diff --git a/ghost/collections/src/CollectionsPostsRepositoryInMemory.ts b/ghost/collections/src/CollectionsPostsRepositoryInMemory.ts
new file mode 100644
index 0000000000..99317b8fa8
--- /dev/null
+++ b/ghost/collections/src/CollectionsPostsRepositoryInMemory.ts
@@ -0,0 +1,17 @@
+import {InMemoryRepository} from '@tryghost/in-memory-repository';
+import {CollectionPost} from './CollectionPost';
+
+export class CollectionsPostsRepositoryInMemory extends InMemoryRepository<string, CollectionPost> {
+    constructor() {
+        super();
+    }
+
+    protected toPrimitive(entity: CollectionPost): object {
+        return {
+            id: entity.id,
+            post_id: entity.postId,
+            collection_id: entity.collectionId,
+            sort_order: entity.sortOrder
+        };
+    }
+}
diff --git a/ghost/collections/src/CollectionsRepositoryInMemory.ts b/ghost/collections/src/CollectionsRepositoryInMemory.ts
index 0904ccf507..ad3a8295ce 100644
--- a/ghost/collections/src/CollectionsRepositoryInMemory.ts
+++ b/ghost/collections/src/CollectionsRepositoryInMemory.ts
@@ -1,9 +1,18 @@
 import {InMemoryRepository} from '@tryghost/in-memory-repository';
 import {Collection} from './Collection';
+import {CollectionPost} from './CollectionPost';
 
 export class CollectionsRepositoryInMemory extends InMemoryRepository<string, Collection> {
-    constructor() {
+    collectionsPostsRepository: any;
+
+    constructor(deps: any) {
         super();
+
+        this.collectionsPostsRepository = deps.collectionsPostsRepository;
+    }
+
+    async saveCollectionPost(collectionPost: CollectionPost) {
+        await this.collectionsPostsRepository.save(collectionPost);
     }
 
     protected toPrimitive(entity: Collection): object {
diff --git a/ghost/collections/src/CollectionsService.ts b/ghost/collections/src/CollectionsService.ts
index 38b0e96555..165a050a0d 100644
--- a/ghost/collections/src/CollectionsService.ts
+++ b/ghost/collections/src/CollectionsService.ts
@@ -1,5 +1,6 @@
 import {Collection} from './Collection';
 import {PostDTO} from './PostDTO';
+import {CollectionPost} from './CollectionPost';
 
 type CollectionsServiceDeps = {
     collectionsRepository: any;
@@ -8,6 +9,13 @@ type CollectionsServiceDeps = {
     }
 };
 
+type CollectionPostDTO = {
+    id?: string;
+    post_id: string;
+    collection_id: string;
+    sort_order?: number;
+};
+
 export class CollectionsService {
     collectionsRepository: any;
     postsRepository: any;
@@ -32,11 +40,36 @@ export class CollectionsService {
         const postIds = postDTOs.map(post => post.id);
         const posts = await this.postsRepository.getBulk(postIds);
 
-        collection.posts = posts.map((post: any) => post.id);
+        for (const post of posts) {
+            const collectionPost = await CollectionPost.create({
+                post_id: post.id,
+                collection_id: collection.id
+            });
+
+            collection.addPost(collectionPost);
+        }
 
         return collection;
     }
 
+    async addPost(collectionPost: CollectionPostDTO): Promise<any> {
+        const collection = await this.collectionsRepository.getById(collectionPost.collection_id);
+
+        if (!collection) {
+            return null;
+        }
+
+        if (!collection.canAddPost(collectionPost)) {
+            return null;
+        }
+
+        const collectionPostEntity = await CollectionPost.create(collectionPost);
+        collection.addPost(collectionPostEntity);
+        const savedPostCollection = await this.collectionsRepository.saveCollectionPost(collectionPostEntity);
+
+        return savedPostCollection;
+    }
+
     async edit(data: any): Promise<Collection | null> {
         const collection = await this.collectionsRepository.getById(data.id);