The reason this works is because how %Struct{} (and, more generally, any kind of map: %{}) matches.
The idea is that, for structs and maps, you often are only interested in one or a few of the fields. Therefore, matching %{foo: _} will match any map that has a key :foo. Likewise, %{} will match any map at all.
When not matching but constructing a value, then, yes, %{} means ‘empty map’ and %Client{} means ‘empty client (whose struct fields are set to their defaults)’.
The order in which = is used (i.e. which side contains the structure to match, and which side contains the name to bind the matched part to) inside a match does not matter. Some people prefer to do it in front, (client = %Client{}), some in the back (%Client{} = client). Both mean the same.
To change your code so it returns a new client, write this:
case Repo.one(Client) do
%Client{} -> %Client{} # Note that the RHS is an empty client, while the LHS matches _any_ client.
nil -> {:error, "some error message"}
end
