How to group by a key and sum other keys with Ramda? - ramda.js

Question

Suppose I have an array of objects like this:
[
{'prop_1': 'key_1', 'prop_2': 23, 'prop_3': 45},
{'prop_1': 'key_1', 'prop_2': 56, 'prop_3': 10},
{'prop_1': 'key_2', 'prop_2': 10, 'prop_3': 5},
{'prop_1': 'key_2', 'prop_2': 6, 'prop_3': 7}
]
I would like to group by the first property and sum the values of the other properties, resulting in an array like this:
[
{'prop_1': 'key_1', 'prop_2': 79, 'prop_3': 55},
{'prop_1': 'key_2', 'prop_2': 16, 'prop_3': 12}
]
What is the correct way to do this using Ramda?
I have attempted to use the following:
R.pipe(
R.groupBy(R.prop('prop_1')),
R.values,
R.reduce(R.mergeWith(R.add), {})
)
But this sums also the value of 'prop_1'.

Answer 1

You'll need to map the groups, and then reduce each group. Check the values of the currently merged keys. If the value is a Number add the values. If not just return the 1st value:
const { pipe, groupBy, prop, values, map, reduce, mergeWith, ifElse, is, add, identity } = R
const fn = pipe(
groupBy(prop('prop_1')),
values,
map(reduce(
mergeWith(ifElse(is(Number), add, identity)),
{}
))
)
const data = [{"prop_1":"key_1","prop_2":23,"prop_3":45},{"prop_1":"key_1","prop_2":56,"prop_3":10},{"prop_1":"key_2","prop_2":10,"prop_3":5},{"prop_1":"key_2","prop_2":6,"prop_3":7}]
const result = fn(data)
console.log(result)
<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.27.0/ramda.js"></script>

Answer 2

If and only if you know the keys in advance, you could opt for something "simpler" (of course YMMV):
reduceBy takes a function similar to Array#reduce
Use prop('prop_1') as a "group by" clause
(It should be relatively straightforward to extract the values out of the object to get the final array.)
console.log(
reduceBy
( (acc, obj) =>
({ prop_1: obj.prop_1
, prop_2: acc.prop_2 + obj.prop_2
, prop_3: acc.prop_3 + obj.prop_3
})
// acc initial value
, { prop_1: ''
, prop_2: 0
, prop_3: 0
}
// group by clause
, prop('prop_1')
, data
)
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.27.0/ramda.min.js"></script>
<script>const {reduceBy, prop} = R;</script>
<script>
const data = [{'prop_1': 'key_1', 'prop_2': 23, 'prop_3': 45}, {'prop_1': 'key_1', 'prop_2': 56, 'prop_3': 10}, {'prop_1': 'key_2', 'prop_2': 10, 'prop_3': 5}, {'prop_1': 'key_2', 'prop_2': 6, 'prop_3': 7}];
</script>

Answer 3

This answer is not as simple as that from Ori Drori. And I'm not sure whether that's a good thing. This seems to more closely fit the requirements, especially if "and sum the values of the other properties" is a simplification of actual requirements. This tries to keep the key property and then combine the others based on your function
const data = [
{'prop_1': 'key_1', 'prop_2': 23, 'prop_3': 45},
{'prop_1': 'key_1', 'prop_2': 56, 'prop_3': 10},
{'prop_1': 'key_2', 'prop_2': 10, 'prop_3': 5},
{'prop_1': 'key_2', 'prop_2': 6, 'prop_3': 7}
]
const transform = (key, combine) => pipe (
groupBy (prop (key)),
map (map (omit ([key]))),
map (combine),
toPairs,
map (([key, obj]) => ({prop_1: key, ...obj}))
)
console .log (
transform ('prop_1', reduce(mergeWith (add), {})) (data)
)
.as-console-wrapper {max-height: 100% !important; top: 0}
<script src="//cdnjs.cloudflare.com/ajax/libs/ramda/0.27.0/ramda.js"></script>
<script> const {pipe, groupBy, prop, map, omit, reduce, mergeWith, add, toPairs, lift, merge, head, objOf, last} = R </script>
If you have a fetish for point-free code that last line could be written as
map (lift (merge) (pipe (head, objOf(key)), last))
But as we are already making points of key and obj, I see no reason. Yes, we could change that, but I think it would become pretty ugly code.
There might well be something to be said for a more reduce-like version, where instead of passing such a combine function, we pass something that combines two values as well as a way to get the initial value. That's left as an exercise for later.