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

[nelsonic]

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):

Travis CI

But once the defimpl is added the tests fail:

Travis CI

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

Code Snapshot when these 5 lines were added:
https://github.com/dwyl/learn-phoenix-framework/pull/42/commits/db72acbe83b184cd6ce3fe342ee51c3e39eb5de8

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://forum.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!

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: http://stackoverflow.com/questions/42609234/defimpl-phoenix-param-to-override-to-param-causes-functionclauseerror-no-funct

[josevalim]

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

[nelsonic]

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

https://github.com/dwyl/learn-phoenix-framework/pull/42/commits/27c4a115fa9cac4429fa31ce53203931586f52f4

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