Defimpl Phoenix.Param to override to_param causes (FunctionClauseError) no function clause matching in Phoenix.Param.Rumbl.Video.to_param/1

Phoenix Forum Questions / Help
1

while trying to follow the examples in Programming Phoenix (book) release P1.0,

everything is working fine until Page 167 when instructed to add the lines:

defimpl Phoenix.Param, for: Rumbl.Video do
  def to_param(%{slug: slug, id: id}) do
    "#{id}-#{slug}"
  end
end

to: rumbl/web/models/video.ex

Before I add the lines, the tests pass (as expected):

But once the defimpl is added the tests fail:

The error message for two of the failing tests is:

** (FunctionClauseError) no function clause matching in Phoenix.Param.Rumbl.Video.to_param/1

Stack trace: Travis CI

image
image1050×463 86.6 KB

Code Snapshot when these 5 lines were added:

Any help with understanding this would be much appreciated!

Note: I’ve tried googling (for a while) 


I’ve read through dozens of similar-looking questions on SO & https://elixirforum.com 


but none were helpful. it appears I’m the only one seeing this error!

Posting a StackOverflow question is my “last resort” before giving up programming! :frowning:

On the “Programming Phoenix” book’s errata page: https://pragprog.com/titles/phoenix/errata there is another person reporting an issue with the same code but their error is different 


Also posted on StackOverflow just in case: web development server - defimpl Phoenix.Param to override to_param causes (FunctionClauseError) no function clause matching in Phoenix.Param.Rumbl.Video.to_param/1 - Stack Overflow

1 Like
2

If we look at the error, it doesn’t seem that your Video struct has the slug field. Are you sure it is there?

1 Like
3

Thanks for quick rely @josevalim, you’re correct! (it’s PEBKAC
) :confused:
adding the field :slug, :string Fixed it. :blush:
case closed :white_check_mark:

Thankfully this won’t be an issue for much longer once Models are gone in 1.3
 :wink:
Obrigado José! :thumbsup:

1 Like