How to map and reduce List of Maps in Elixir - functional-programming

Question

What's a good way to map and reduce the elixir's list and convert it into new list.
Requirement:
1. Find the maps with same id:
2. Merge the values of "role" key (i.e. Gather all the unique values).
3. For all other maps(elements of list), do nothing.
list = [%{"id": 1, "role": ["A", "B"]}, %{"id": 2, "role": ["B", "C"]}, %{"id": 1, "role": ["C", "A"]} ]
needs to be converted in the following list:
ans_list = [%{"id": 1, "role": ["A", "B", "C"]}, %{"id": 2, "role": ["B", "C"]}]

Answer 1

You can use Enum.group_by/2 to group by the id, then for each group, pass the role to Enum.flat_map/2 and Enum.uniq/1:
list = [%{"id": 1, "role": ["A", "B"]}, %{"id": 2, "role": ["B", "C"]}, %{"id": 1, "role": ["C", "A"]} ]
list
|> Enum.group_by(&(&1.id))
|> Enum.map(fn {key, value} ->
%{id: key, role: value |> Enum.flat_map(&(&1.role)) |> Enum.uniq}
end)
|> IO.inspect
Output:
[%{id: 1, role: ["A", "B", "C"]}, %{id: 2, role: ["B", "C"]}]
As requested in a comment below, here's how to preserve all key/value pairs and only modify role of the first item of a group:
list = [%{"id": 1, "role": ["A", "B"], "somekey": "value of the key 1"},
%{"id": 2, "role": ["B", "C"], "somekey": "value of the key 2"},
%{"id": 1, "role": ["C", "A"], "somekey": "value of the key 3"}]
list
|> Enum.group_by(&(&1.id))
|> Enum.map(fn {_, [value | _] = values} ->
%{value | role: values |> Enum.flat_map(&(&1.role)) |> Enum.uniq}
end)
|> IO.inspect
Output:
[%{id: 1, role: ["A", "B", "C"], somekey: "value of the key 1"},
%{id: 2, role: ["B", "C"], somekey: "value of the key 2"}]