_method param added to a form?

Phoenix Forum Questions / Help
1

Under what circumstances a _method field is added to a form, so that Plug.MethodOverride can use it to turn a POST into, for example, a PUT? Sometimes a hidden input field is automatically generated, other times I need to add <input name="_method" type="hidden" value="put" /> to the form. I’m clearly missing something…

2

I would say it’s added whenever your form uses a method other than POST or GET. Web forms can only send data using those two methods (<form>: The Form element - HTML: HyperText Markup Language | MDN), other methods have to be simulated.

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3

IIRC form_for decides on a method based on the action of the changeset it’s passed (put for update vs plain post for insert) - can you be more specific about how you’re rendering these forms?

4

Here is how I render the form:

<%= form_for @changeset, Routes.some_path(@conn, :update, abc), fn _f -> %>
    <input name="_method" type="hidden" value="put" />
    <%= submit("update", class: "btn") %>
<% end %>

if <input name="_method" type="hidden" value="put" /> is removed, no _method will be sent in the POST, independently of the action passed to the changeset (I tried bot update and insert).

Any idea why?

5

The action in the changeset is irrelevant, you need to pass the :method option to form_for: Phoenix.HTML.Form — Phoenix.HTML v2.14.2

Quoting the docs:

:method - the HTTP method. If the method is not “get” nor “post”, an input tag with name _method is generated along-side the form tag. Defaults to “post”.

And you don’t need to manually render the _method_ hidden field

6

The action is indeed irrelevant, but the schema metadata on a struct used for a changeset is used to infer the method for the form if not explicitly supplied. If the metadata state is :built (or anything else) is uses "post", if it’s :loaded it infers "put".

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7

That’s what I needed, although @LostKobrakai 's reply just made me learn something new. Many thanks to both of you!