Rails 5: form_for vs form_with

Use form_with (more up-to-date)

• form_with is the latest.
• form_for and form_tag are obsolete.

Why is there a change?

See Kasper Timm Hansen's pull request - I cannot state it better than pull request itself:

form_tag and form_for serve very similar use cases. This PR unifies that usage such that form_with can output just the opening form tag akin to form_tag and can just work with a url, for instance.

This means you don't need to use form_tag if you don't have a model. You can use the form_with helper and it can still handle URLs.

Note Re: Ids and Classes: The Ruby 5.1 version of form_with, by default doesn't attach a class or id to the form. For Rails 5.2 and later, however, form_with WILL automatically generate ids based on the model, if present—basically the same id behavior as form_for, so you will NOT need to manually specify them anymore. (source)

This is really in preparation for rails 5.1 where only form_with should be used. It's meant to serve as a replacement for the two methods form_for and form_tag.

form_for and form_tag in Rails were very similar, both allowed you to create a form tag but the first one uses model’s attributes to build create or update form, while the second one simply creates an HTML form tag with the passed URL as action.

Existing answers are great. What I also found helpful were the first few paragraphs here. Basically:

form_tag and form_for are soft deprecated and they will be replaced by form_with in the future.

• DHH's proposal: https://github.com/rails/rails/issues/25197
• The pull request: https://github.com/rails/rails/pull/26976/files (I didn't read it, but it could be useful)