Facebook Sign-in using Clojure

In order to provide Facebook sign-in functionality you need to create a new Facebook App from this url: https://developers.facebook.com/docs/facebook-login. Provide the redirection URI that you’ll use for the App. After setting up the app you’ll get an App ID and App Secret key.

We’ll use “clj-oauth2.client” to redirect a user to the authentication page, “clj-http.client” for HTTP requests, “cheshire.core” to parse Json string in order to get keywords, “noir.respose” to provide re-directions and “compojure.core” for routing.

We’ll begin with defining a new namespace:

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(ns exampleapp.routes.facebook (:use compojure.core) (:require [clj-oauth2.client :as oauth2] [noir.response :as resp] [clj-http.client :as client] [cheshire.core :as parse]))

In facebook namespace define an atom, facebook-user, to store Facebook user details:

1 2	(def facebook-user (atom {:facebook-id "" :facebook-name "" :facebook-email ""}))

Now include the App ID and App Secret key you just got after creating a new Facebook app along with the redirection URI, in the code below:

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(def APP_ID "your app id") (def APP_SECRET "your app secret key") (def REDIRECT_URI "http://localhost:3000/auth_facebook")

and define an oauth2 map containing all the details required for Facebook log in:

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(def facebook-oauth2 {:authorization-uri "https://graph.facebook.com/oauth/authorize" :access-token-uri "https://graph.facebook.com/oauth/access_token" :redirect-uri REDIRECT_URI :client-id APP_ID :client-secret APP_SECRET :access-query-param :access_token :scope ["email"] :grant-type "authorization_code"})

We can use “make-auth-request” function defined in “clj-oauth2.client” library to get the redirect URI on which the user should be redirected. The user can now give access to the app we created in first step.

1 2	(resp/redirect (:uri (oauth2/make-auth-request facebook-oauth2)))

Note that we used noir.response/redirect function to provide the redirection.

After a Facebook user grant access to the app, we’ll get a response containing access token on the redirection URI we provided while setting up our Facebook app. We’ll use “compojure.core’s” GET to get the response:

1 2	(defroutes facebook-routes (GET "/auth_facebook" {params :query-params} (facebook params)))

and pass it to a function in order to get the access token and user details.

We’ll use the code below for getting access token and user details:

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(defn facebook [params] (let [access-token-response (:body (client/get (str "https://graph.facebook.com/oauth/access_token?" "client_id=" APP_ID "&redirect_uri=" REDIRECT_URI "&client_secret=" APP_SECRET "&code=" (get params "code")))) access-token (get (re-find #"access_token=(.*?)&expires=" access-token-response) 1) user-details (-> (client/get (str "https://graph.facebook.com/me?access_token=" access-token)) :body (parse/parse-string))] (swap! facebook-user #(assoc % :facebook-id %2 :facebook-name %3 :facebook-email %4) (get user-details "id") (get user-details "first_name") (get user-details "email"))))

This article was first published on the Knoldus blog.