How to check for a condition inside a pipe chain?

Do you have an example of the code? I guess that with the code is a little bit better to understand what you mean.

I have discovered a possible solution.. Here is the working pipeline with conditionals:

  page =
  Policy
    |> join(:left, [p], c in Client, p.client_id == c.id)
    |> where([p, c], c.user_id == ^conn.assigns.current_user.id)

  # filter by expires
  if is_nil(params["policy"]["expires"]) do
    expires_params = "01-01-1970" |> Timex.parse!("%d-%m-%Y", :strftime) |> Ecto.DateTime.cast!
    page = page |> where([p, c], p.expires >= ^expires_params)
  else
    page = page |> where([p, c], p.expires >= ^params["policy"]["expires"])
  end

  # filter by client_id
  if is_list(params["policy"]["client_id"]) do
    page = page |> where([p, c], p.client_id in ^params["policy"]["client_id"])
  end

  # final processing
  page = page |> preload([p, c], [client: c]) |> Repo.paginate(params)

Works, but maybe there is a better / nicer way to code it

Actually, you should never assign variables inside conditionals like if, case and so on.

Instead of writing:

if is_list(params["policy"]["client_id"]) do
  page = page |> where([p, c], p.client_id in ^params["policy"]["client_id"])
end

You should write:

page = 
  if is_list(params["policy"]["client_id"]) do
    page |> where([p, c], p.client_id in ^params["policy"]["client_id"])
  else
    page
  end

Other than that I think it’s a good solution, but you could also take a look at Ecto’s 2.1 new dynamic expressions: ecto/CHANGELOG.md at v2.1 · elixir-ecto/ecto · GitHub

It might be possible to extract a function where you can pattern match and compose the query. Here’s what it could look like for the expires policy (actual code may depend on where your functions and usage are located):

# Policy module
def with_expiry(page, nil) do
  expires_params = "01-01-1970" |> Timex.parse!("%d-%m-%Y", :strftime) |> Ecto.DateTime.cast!
  page |> where([p, c], p.expires >= ^expires_params)
end
def with_expiry(page, expires_params) do
  page |> where([p, c], p.expires >= ^expires_params)
end

#...

page =
  Policy
    |> join(:left, [p], c in Client, p.client_id == c.id)
    |> where([p, c], c.user_id == ^conn.assigns.current_user.id)
    |> Policy.with_expiry(params["policy"]["expires"])
    # ...

@rossta, @vidalraphael, @kelvinst

Thank you very much.. I have learned new things here today.. I am happy that I have found my way back to this forum after some bad bi yearly banning experiences at holy stackoverflow (latest only because I asked a duplicate question)..

Yep! @rossta thank you! That’s the way to go!

Hi there!

If I have to pass in a limit clause based on a function argument. Is function pattern matching the best way to do this? In the solution above, there is 2 functions that returns where clause. Is it possible to do

def with_limit(page, nil) do
page
end

def with_limit(page, limit_size) do
page |> limit: size
end

query |> with_limit limit_size

i was going to do a
if param do
page = query |> limit: size
end

but someone mentioned earlier that i shouldnt be assigning in if/else clause.

I’m often lazy and just put a case between things… >.>

value
|> something(42)
|> something_else()
|> case do
     page where limit_size == nil -> page
     page -> page |> limit(size)
   end
|> some_more_things()
...

Also you really need to use code tags instead of quotes for bodies of code, you do it like:
```elixir
Code goes here like 2+2
```
Makes:

Code goes here like 2+2

heheh thanks! you saved me again man! will try it out!

IO.puts "Thanks!"

Hehe, do note the ‘proper’ way to do it is different function heads and pipe to the function, but when lazy… ^.^;