Struct to struct mapping

Questions / Help
1

Hi All.

is there a simple way to take an existing struct and map certain field values into a different second struct? My use case is I’d like to write a “log” file based on some key fields from a Plug Conn. I could just copy over the relevant fields or convert to a map and then to the second struct but I’m not sure if there is a better way.

cheers

Dave

2

That sounds fine, probably a single line anyway, could even look down into the ‘conn’ to pull out specific sub-values to put in your new struct by using get_in. :slight_smile:

3

Thanks @OvermindDL1 - is there any real performance overhead from converting and copying?

cheers

Dave

4

As long as you don’t ‘change’ the data as you copy it, nope, everything is immutable so quite literally they just point to the same bit of memory. :slight_smile:

5

What do you think about this:

defmodule Mapper do
  def map(struct1, struct2) do
    struct struct2, Map.from_struct(struct1)
  end
end

Here are some tests:

iex(1)> defmodule Struct1 do
...(1)>   defstruct one: 1, two: 2, three: 3, four: 4
...(1)> end
{:module, Struct1,
 <<70, 79, 82, 49, 0, 0, 9, 80, 66, 69, 65, 77, 69, 120, 68, 99, 0, 0, 0, 186,
   131, 104, 2, 100, 0, 14, 101, 108, 105, 120, 105, 114, 95, 100, 111, 99, 115,
   95, 118, 49, 108, 0, 0, 0, 4, 104, 2, ...>>,
 %Struct1{four: 4, one: 1, three: 3, two: 2}}
iex(2)> defmodule Struct2 do
...(2)>   defstruct one: 2, two: 2, five: 5
...(2)> end
{:module, Struct2,
 <<70, 79, 82, 49, 0, 0, 9, 64, 66, 69, 65, 77, 69, 120, 68, 99, 0, 0, 0, 186,
   131, 104, 2, 100, 0, 14, 101, 108, 105, 120, 105, 114, 95, 100, 111, 99, 115,
   95, 118, 49, 108, 0, 0, 0, 4, 104, 2, ...>>,
 %Struct2{five: 5, one: 2, two: 2}}
iex(3)> defmodule Mapper do
...(3)>   def map(struct1, struct2) do
...(3)>     struct struct2, Map.from_struct(struct1)
...(3)>   end
...(3)> end
{:module, Mapper,
 <<70, 79, 82, 49, 0, 0, 5, 80, 66, 69, 65, 77, 69, 120, 68, 99, 0, 0, 0, 169,
   131, 104, 2, 100, 0, 14, 101, 108, 105, 120, 105, 114, 95, 100, 111, 99, 115,
   95, 118, 49, 108, 0, 0, 0, 4, 104, 2, ...>>, {:map, 2}}
iex(4)> Mapper.map(%Struct2{}, %Struct1{})
%Struct1{four: 4, one: 2, three: 3, two: 2}
iex(5)> Mapper.map(%Struct1{}, %Struct2{})
%Struct2{five: 5, one: 1, two: 2}
iex(6)> Mapper.map(%Struct1{one: 42, two: 99}, %Struct2{})
%Struct2{five: 5, one: 42, two: 99}
iex(7)>
6

thanks @smpallen99 - in the end I simply copied the relevant fields but I’ll put your option in my toolset for later.

cheers

Dave

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