# `JSONAPI.UnderscoreParameters`
[🔗](https://github.com/beam-community/jsonapi/blob/main/lib/jsonapi/plugs/underscore_parameters.ex#L1)

Takes dasherized JSON:API params and converts them to underscored params. Add
this to your API's pipeline to aid in dealing with incoming parameters such as query
params or data.

By default the newly underscored params will only replace the existing `params` field
of the `Plug.Conn` struct, but leave the `query_params` and `body_params` untouched.
If you are using the `JSONAPI.QueryParser` and need to also have the `query_params` on
the `Plug.Conn` updated, set the `replace_query_params` option to `true`.

Note that this Plug will only underscore parameters when the request's content
type is for a JSON:API request (i.e. "application/vnd.api+json"). All other
content types will be ignored.

## Options

  * `:replace_query_params` - When `true`, it will replace the `query_params` field in
  the `Plug.Conn` struct.  This is useful when you have downstream code which is
  expecting underscored fields in `query_params`, and not just in `params`. Defaults
  to `false`.

## Example

%{
  "data" => %{
    "attributes" => %{
      "foo-bar" => true
    }
  }
}

are transformed to:

%{
  "data" => %{
    "attributes" => %{
      "foo_bar" => true
    }
  }
}

Moreover, with a GET request like:

    GET /example?filters[dog-breed]=Corgi

**Without** this Plug your index action would look like:

    def index(conn, %{"filters" => %{"dog-breed" => "Corgi"}})

And **with** this Plug:

    def index(conn, %{"filters" => %{"dog_breed" => "Corgi"}})

Your API's pipeline might look something like this:

    # e.g. a Phoenix app

    pipeline :api do
      plug JSONAPI.EnforceSpec
      plug JSONAPI.UnderscoreParameters
    end

---

*Consult [api-reference.md](api-reference.md) for complete listing*