Prevent API error when NULL bio is given

app/models/user_detail.rb

@@ -9,6 +9,8 @@ class UserDetail < ApplicationRecord
   validates :job_title, length: { maximum: 200 }
   validates :bio, length: { maximum: 255 }, allow_blank: true
 
+  before_save :prevent_nil_bio
+
   cache_markdown_field :bio
 
   def bio_html
@@ -22,4 +24,10 @@ class UserDetail < ApplicationRecord
   def invalidated_markdown_cache?
     self.class.column_names.include?('bio_html') && super
   end
+
+  private
+
+  def prevent_nil_bio
+    self.bio = '' if bio_changed? && bio.nil?
+  end
 end

changelogs/unreleased/227863-fix-api-error-on-null-bio.yml

+---
+title: Fix API errors when null value is given for the bio
+merge_request: 36650
+author:
+type: fixed

doc/api/users.md

@@ -89,8 +89,8 @@ GET /users
     "web_url": "http://localhost:3000/john_smith",
     "created_at": "2012-05-23T08:00:58Z",
     "is_admin": false,
-    "bio": null,
-    "bio_html": null,
+    "bio": "",
+    "bio_html": "",
     "location": null,
     "skype": "",
     "linkedin": "",
@@ -129,8 +129,8 @@ GET /users
     "web_url": "http://localhost:3000/jack_smith",
     "created_at": "2012-05-23T08:01:01Z",
     "is_admin": false,
-    "bio": null,
-    "bio_html": null,
+    "bio": "",
+    "bio_html": "",
     "location": null,
     "skype": "",
     "linkedin": "",
@@ -247,8 +247,8 @@ Parameters:
   "avatar_url": "http://localhost:3000/uploads/user/avatar/1/cd8.jpeg",
   "web_url": "http://localhost:3000/john_smith",
   "created_at": "2012-05-23T08:00:58Z",
-  "bio": null,
-  "bio_html": null,
+  "bio": "",
+  "bio_html": "",
   "location": null,
   "public_email": "john@example.com",
   "skype": "",
@@ -283,8 +283,8 @@ Example Responses:
   "web_url": "http://localhost:3000/john_smith",
   "created_at": "2012-05-23T08:00:58Z",
   "is_admin": false,
-  "bio": null,
-  "bio_html": null,
+  "bio": "",
+  "bio_html": "",
   "location": null,
   "public_email": "john@example.com",
   "skype": "",
@@ -372,6 +372,9 @@ over `password`. In addition, `reset_password` and
 NOTE: **Note:**
 From [GitLab 12.1](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/29888/), `private_profile` will default to `false`.
 
+NOTE: **Note:**
+From [GitLab 13.2](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35604), `bio` will default to `""` instead of `null`.
+
 ```plaintext
 POST /users
 ```
@@ -503,8 +506,8 @@ GET /user
   "avatar_url": "http://localhost:3000/uploads/user/avatar/1/index.jpg",
   "web_url": "http://localhost:3000/john_smith",
   "created_at": "2012-05-23T08:00:58Z",
-  "bio": null,
-  "bio_html": null,
+  "bio": "",
+  "bio_html": "",
   "location": null,
   "public_email": "john@example.com",
   "skype": "",
@@ -553,8 +556,8 @@ GET /user
   "web_url": "http://localhost:3000/john_smith",
   "created_at": "2012-05-23T08:00:58Z",
   "is_admin": false,
-  "bio": null,
-  "bio_html": null,
+  "bio": "",
+  "bio_html": "",
   "location": null,
   "public_email": "john@example.com",
   "skype": "",

spec/requests/api/users_spec.rb

@@ -910,6 +910,14 @@ RSpec.describe API::Users, :do_not_mock_admin_mode do
       expect(user.reload.bio).to eq('')
     end
 
+    it 'updates user with nil bio' do
+      put api("/users/#{user.id}", admin), params: { bio: nil }
+
+      expect(response).to have_gitlab_http_status(:ok)
+      expect(json_response['bio']).to eq('')
+      expect(user.reload.bio).to eq('')
+    end
+
     it "updates user with new password and forces reset on next login" do
       put api("/users/#{user.id}", admin), params: { password: '12345678' }