Summary
I am currently trying to get a better understanding of elixir macros and in doing so encountered the section about dynamic queries in the ecto documentation. The example, however, leads to an unwanted true and part in the ecto query where and I am trying to have a look at how to solve that.
The trimmed down documentation example
So I tried out the example given and it seems to add a superfluous true and in front of the where clause. I trimmed the example down and put it inside a testcase:
defmodule MyMacroTest do
use ExUnit.Case, async: true
import Ecto.Query
def filter_where(params) do
Enum.reduce(params, dynamic(true), fn
{"author", value}, dynamic ->
dynamic([p], ^dynamic and p.author == ^value)
end)
end
@q from(p in "post")
test "author" do
query = %{"author" => "foo"}
assert inspect(where(@q, ^filter_where(query))) ==
inspect(where(@q, [p], p.author == ^query["author"]))
end
end
which fails with
Assertion with == failed
code: assert inspect(where(@q, ^filter_where(query))) == inspect(where(@q, [p], p.author == ^query["author"]))
left: "#Ecto.Query<from p0 in \"post\", where: true and p0.author == ^\"foo\">"
right: "#Ecto.Query<from p0 in \"post\", where: p0.author == ^\"foo\">"
The fancy if statement 
Switching out the dynamic(true) within the call to Enum.reduce/3 with nil and using if seems to work
defmodule MyMacro2Test do
use ExUnit.Case, async: true
import Ecto.Query
def filter_where(params) do
Enum.reduce(params, nil, fn
{"author", value}, dynamic ->
if dynamic do
dynamic([p], ^dynamic and p.author == ^value)
else
dynamic([p], p.author == ^value)
end
end)
end
@q from(p in "post")
test "author" do
query = %{"author" => "foo"}
assert inspect(where(@q, ^filter_where(query))) ==
inspect(where(@q, [p], p.author == ^query["author"]))
end
end
Finished in 0.04 seconds (0.04s async, 0.00s sync)
1 test, 0 failures
Even though it works, it is not a thing of beauty and given that the dynamic macro basically outputs AST similar to what I would like, writing another macro appears to be the correct thing to do.
But here is the part I seem to struggle with
I now don’t know how to exactly extract that conditional into a macro. I guess I will need a macro because I need to wrap the dynamic macro and pass through the arguments in the same syntax.
Currently stuck with this version
defmodule MyMacroTest3 do
use ExUnit.Case, async: true
import Ecto.Query
defmacro dynamic_and(dynamic, bindings, expr) do
quote do
if unquote(dynamic) do
dynamic(unquote(bindings), unquote(dynamic) and unquote(expr))
else
dynamic(unquote(bindings), unquote(expr))
end
end
end
def filter_where(params) do
Enum.reduce(params, nil, fn
{"author", value}, dynamic ->
dynamic_and(^dynamic, [p], p.author == ^value)
end)
end
@q from(p in "post")
test "author" do
query = %{"author" => "foo"}
assert inspect(where(@q, ^filter_where(query))) ==
inspect(where(@q, [p], p.author == ^query["author"]))
end
end
which gives me a compilation error about a misplaced operator ^dynamic
The pin operator ^ is supported only inside matches or inside custom macros. Make sure you are inside a match or all necessary macros have been required
│
18 │ dynamic_and(^dynamic, [p], p.author == ^value)
I tried multiple variations of pinning and unquote/not unquote but I somehow currently fail to see where to look next for the correct way of solving this.
I also thought that a potential solution could involve returning some AST directly, or maybe it would be better to try to construct an %Ecto.Query.DynamicExpr struct myself.
Also got myself a copy of that Metaprogramming book and I am currently reading it hoping that it might help, and even got some bots involved 
Anyway, I would really apreciate any pointers that could be thrown at me and thank you very much in advance! 
).
