400 Bad Request in Ruby on Rails: A complete guide to causes and solutions

　Facing the “400 Bad Request” error while developing with Ruby on Rails is a common problem for developers. This error occurs when the server refuses to process a request from a client because it is malformed or incomplete. Let’s take a closer look at the causes and solutions if you run into this problem in your Rails application.

Cause 1: Improper form configuration

　Improperly configured forms are a common cause of 400 errors. For example, a parameter is not set properly or a form input field is incorrect. This will cause a 400 error because the server cannot receive the expected parameters.

solution:

– Make sure each input field on the form is set correctly.

– Use Rails’ helper methods such as `form_for` and `form_with` to properly construct the form.

Cause 2: CSRF token mismatch

　Rails uses CSRF tokens for added security. When a form request is sent to the server, if the CSRF token is missing or mismatched, Rails will reject the request and return a 400 Bad Request error.

solution:

– Verify that the correct CSRF token is included in the form.

– Automatically manage CSRF tokens by including Rails’ `csrf_meta_tags` in your layout.

Cause 3: Missing or incorrect parameters

　A 400 error will also occur if a required parameter in the request is missing or incorrect. For example, a required parameter is missing from the request, or the parameter name or value is incorrect.

solution:

– Make sure the required parameters are included in the request.

– Check that the parameter names and values ​​are correct.

Additional considerations

Encoding problem:

　A 400 error may occur if the encoding of the request is different from the format expected by the server. Applications that include multilingual support are especially prone to encoding problems.

solution:

– Make sure your application and server use the appropriate encoding format.

– Specify appropriate `Content-Type` and character encoding in request and response headers.

Network issues:

　Sometimes, 400 errors can be caused by network issues on the client side. Requests may be marked as incomplete or malformed due to factors such as an unstable network connection or a misconfigured proxy server.

solution:

– Check your network connection and fix it if necessary.

– Check your proxy settings and firewall settings to ensure there are no issues.

Related Links

– [Action Controller 概览— Ruby on Rails Guides]

– [「400 Bad Request」エラーの解決方法（原因と対策） – Kinsta]

– [HTTP Error 400 Bad request 的问题分析解决 – 腾讯云]