Learn how to implement CRUD operations for a blog platform using Clojure and MongoDB, focusing on creating, retrieving, updating, and deleting blog posts and comments with robust error handling and input validation.
In this section, we will delve into implementing CRUD (Create, Read, Update, Delete) operations for a blog platform using Clojure and MongoDB. This will involve creating new blog posts and comments, retrieving posts with their comments, updating content, and deleting entries. We’ll emphasize error handling and input validation to maintain data integrity throughout the process.
Before we begin coding, ensure you have MongoDB installed and running on your machine. You can download MongoDB from MongoDB’s official website. Additionally, make sure your Clojure development environment is set up with Leiningen, as detailed in Appendix A of this book.
Let’s start by defining the data model for our blog post. In MongoDB, we’ll use a collection named posts to store our blog entries. Each post will have a title, content, author, and a list of comments.
In Clojure, we can represent a blog post using a simple map:
1(defn create-post [title content author]
2 {:title title
3 :content content
4 :author author
5 :comments []})
To insert a new post into MongoDB, we’ll use the Monger library. First, add Monger to your project.clj dependencies:
1:dependencies [[com.novemberain/monger "3.5.0"]]
Now, let’s write a function to insert a new post:
1(ns blog.core
2 (:require [monger.core :as mg]
3 [monger.collection :as mc]))
4
5(defn insert-post [db post]
6 (mc/insert db "posts" post))
7
8(defn create-and-insert-post [db title content author]
9 (let [post (create-post title content author)]
10 (insert-post db post)))
Before inserting a post, it’s crucial to validate the input to ensure data integrity. We can use clojure.spec for this purpose:
1(ns blog.validation
2 (:require [clojure.spec.alpha :as s]))
3
4(s/def ::title (s/and string? #(not (empty? %))))
5(s/def ::content (s/and string? #(not (empty? %))))
6(s/def ::author (s/and string? #(not (empty? %))))
7
8(s/def ::post (s/keys :req [::title ::content ::author]))
9
10(defn validate-post [post]
11 (if (s/valid? ::post post)
12 post
13 (throw (ex-info "Invalid post data" (s/explain-data ::post post)))))
Integrate validation into the insertion process:
1(defn create-and-insert-post [db title content author]
2 (let [post (create-post title content author)]
3 (validate-post post)
4 (insert-post db post)))
To retrieve posts along with their comments, we can query the posts collection:
1(defn get-all-posts [db]
2 (mc/find-maps db "posts"))
For a specific post by title:
1(defn get-post-by-title [db title]
2 (mc/find-one-as-map db "posts" {:title title}))
Comments are stored as a list within each post document. To add a comment, we need to update the post document:
1(defn add-comment [db post-title comment]
2 (mc/update db "posts" {:title post-title}
3 {$push {:comments comment}}))
Similar to posts, comments should be validated:
1(s/def ::comment (s/and string? #(not (empty? %))))
2
3(defn validate-comment [comment]
4 (if (s/valid? ::comment comment)
5 comment
6 (throw (ex-info "Invalid comment data" (s/explain-data ::comment comment)))))
Integrate validation:
1(defn add-comment [db post-title comment]
2 (validate-comment comment)
3 (mc/update db "posts" {:title post-title}
4 {$push {:comments comment}}))
Updating a post involves modifying its content or title. Here’s how you can update a post’s content:
1(defn update-post-content [db post-title new-content]
2 (mc/update db "posts" {:title post-title}
3 {$set {:content new-content}}))
Ensure the new content is validated:
1(defn update-post-content [db post-title new-content]
2 (validate-post {:title post-title :content new-content :author "dummy"})
3 (mc/update db "posts" {:title post-title}
4 {$set {:content new-content}}))
To delete a post, simply remove it from the collection:
1(defn delete-post [db post-title]
2 (mc/remove db "posts" {:title post-title}))
Let’s put it all together in a comprehensive example:
1(ns blog.core
2 (:require [monger.core :as mg]
3 [monger.collection :as mc]
4 [blog.validation :refer [validate-post validate-comment]]))
5
6(defn create-post [title content author]
7 {:title title
8 :content content
9 :author author
10 :comments []})
11
12(defn insert-post [db post]
13 (mc/insert db "posts" post))
14
15(defn create-and-insert-post [db title content author]
16 (let [post (create-post title content author)]
17 (validate-post post)
18 (insert-post db post)))
19
20(defn get-all-posts [db]
21 (mc/find-maps db "posts"))
22
23(defn get-post-by-title [db title]
24 (mc/find-one-as-map db "posts" {:title title}))
25
26(defn add-comment [db post-title comment]
27 (validate-comment comment)
28 (mc/update db "posts" {:title post-title}
29 {$push {:comments comment}}))
30
31(defn update-post-content [db post-title new-content]
32 (validate-post {:title post-title :content new-content :author "dummy"})
33 (mc/update db "posts" {:title post-title}
34 {$set {:content new-content}}))
35
36(defn delete-post [db post-title]
37 (mc/remove db "posts" {:title post-title}))
38
39(defn -main []
40 (let [conn (mg/connect)
41 db (mg/get-db conn "blog")]
42 (create-and-insert-post db "My First Post" "This is the content of my first post." "Author Name")
43 (add-comment db "My First Post" "This is a comment.")
44 (println (get-all-posts db))
45 (update-post-content db "My First Post" "Updated content.")
46 (delete-post db "My First Post")))
try-catch blocks to handle exceptions during database operations. Log errors for debugging purposes.Implementing CRUD operations for a blog platform using Clojure and MongoDB involves understanding the data model, performing database operations, and ensuring data integrity through validation and error handling. By following the practices outlined in this section, you can build a robust and scalable blog application.