Enum.sort_by list of maps

grangerelixx

I have a list of maps.

post_a =
%{
post_body_size: 4,
date: ~D[2021-06-05]
}

post_b =
%{
post_body_size: 4,
date: ~D[2021-06-04]
}

post_c =
%{
post_body_size: 4,
date: ~D[2021-06-03]
}

post_d =
%{
post_body_size: 5,
date: ~D[2021-06-02]
}

I am trying to organize the posts based on the body size (ascending order) and for cases when body sizes are same, the next sort_by is the date(descending order).

[post_count_1, post_count_2, post_count_3, post_count_4] =
        [post_a, post_b, post_c, post_d]
        |> Enum.sort_by(&{Map.fetch(&1, :post_body_size), {:desc, Map.fetch(&1, :date)}})

This doesn’t seem to sort the posts with (only) same body size according to date. Any inputs on how to resolve this or where I am possibly going wrong?

15 comments

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2021-07-14 18:02:25 UTC

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APB9785

Creator of ECSx

In order to compare Date structs, you have to pass the Date module as the third argument to Enum.sort_by/3 as shown in the very last example from Enum - sort_by/3. But by doing this I’m not sure if you can do the Integer comparison for :post_body_size in the same function. Here’s a two-pass implementation:

post_list
|> Enum.sort_by(&Map.fetch!(&1, :date), {:desc, Date})
|> Enum.sort_by(&Map.fetch!(&1, :post_body_size))

Post #1

grangerelixx

Thank you so much This worked for when I want to sort the post by post_body_size ascending and date desc. I also have another condition where I want to order post_body_size by descending where the integer comparison doesn’t work

Post #2

APB9785

Creator of ECSx

You can sort :post_body_size like

...
|> Enum.sort_by(&Map.fetch!(&1, :post_body_size), :desc)

and it will be stable because :desc is a convenience for >=/2

Post #3

grangerelixx

I tried this but it doesn’t order correctly

Post #4

APB9785

Creator of ECSx

What exactly are the sort conditions?

Post #5

grangerelixx

I have four posts and I want to able to order it according to both asc and desc post_body_size. The default date order is desc so if the post_body_sizes are same, it defaults to order according to date desc(only the posts that have same post_body_size). I need the top two posts in both the cases.

This is for test.

Post #6

APB9785

Creator of ECSx

I’m still having a little trouble understanding. Can you show the test code you’ve already written?

Post #7

grangerelixx

I am sorry, its working, I have been trying to assert with the wrong posts. Your solution works for both the cases. Thanks a lot!

Post #8

grangerelixx

Can this be done in a single line? Like

|> Enum.sort_by(& {&1.date, {:desc, Date}, &1.post_body_size, :desc})

This is a wrong syntax but is there a right way to do it?

Post #9

APB9785

Creator of ECSx

Looking at the source for Enum.sort_by/3, I didn’t see any way to do it, because the function expects three arguments:

the Enumerable to be sorted
the mapper, which retrieves the desired elements
the sorter, which compares the elements.

You are able to “overload” the mapper with a tuple of elements, because the sorter will apply to each element in the tuple one-by-one. But the same sorter applies to all elements in the tuple. So in your case, where you want to use two different sorters (>=/2 for Integers, and Date.compare/2 for Dates), I think you must call the function twice.

Post #10

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