How to filter records in a nested array using ramda?

Question

I've seen this question in several places but still can't figure this out. Using ramda, how can I filter the following object to return the records that are true for tomatoes?

[
    {
        "id": "a",
        "name": "fred",
        "food_prefs": {
            "tomatoes": true,
            "spinach": true,
            "pasta": false
        },
        "country": "singapore"
    },
    {
        "id": "b",
        "name": "alexandra",
        "food_prefs": {
            "tomatoes": false,
            "spinach": true,
            "pasta": true
        },
        "country": "france"
    },
    {
        "id": "c",
        "name": "george",
        "food_prefs": {
            "tomatoes": true,
            "spinach": false,
            "pasta": false
        },
        "country": "argentina"
    }
]

Storing this array as myData object, I thought that the following should work:

const R = require("ramda")

const lovesTomatoes = R.pipe ( // taken from: https://stackoverflow.com/a/61480617/6105259
    R.path (["food_prefs"]),
    R.filter (R.prop ("tomatoes"))
)

console.log(lovesTomatoes(myData))

But I end up with the error:

if (typeof obj[methodNames[idx]] === 'function') {

What am I doing wrong?

EDIT

The answers provided by @Ori Drori and @ThanosDi are both great, but I want to emphasize that a pipe-based solution would be ideal because I have follow-up steps I wish to carry on the filtered array. Consider for example the following array. It's similar the one above, but includes more data: year_born and year_record.

[
    {
        "id": "a",
        "name": "fred",
        "year_born": 1995,
        "year_record": 2010,
        "food_prefs": {
            "tomatoes": true,
            "spinach": true,
            "pasta": false
        },
        "country": "singapore"
    },
    {
        "id": "b",
        "name": "alexandra",
        "year_born": 2002,
        "year_record": 2015,
        "food_prefs": {
            "tomatoes": false,
            "spinach": true,
            "pasta": true
        },
        "country": "france"
    },
    {
        "id": "c",
        "name": "george",
        "year_born": 1980,
        "year_record": 2021,
        "food_prefs": {
            "tomatoes": true,
            "spinach": false,
            "pasta": false
        },
        "country": "argentina"
    }
]

So, for example, to answer a full question such as "for those who love tomatoes, what is the average age at the time of the record creation?"

we would need to:

filter the records that love tomates;
extract the elements year_born and year_record
get the difference between values
take the average of the differences

Therefore, using a pipe would be very beneficial.

Ori Drori · Accepted Answer · 2022-02-02 17:44:38Z

2

What went wrong?

You try to get the value of food_prefs out of the array. Since the array doesn't have this key - R.path (["food_prefs"]) is undefined, and then you try to filter this undefined value.

How to solve this problem?

Filter the array, and use R.path to get the tomatoes value.

const { filter, path, identity } = R

const lovesTomatoes = filter(path(['food_prefs', 'tomatoes']))

const data = [{"id":"a","name":"fred","food_prefs":{"tomatoes":true,"spinach":true,"pasta":false},"country":"singapore"},{"id":"b","name":"alexandra","food_prefs":{"tomatoes":false,"spinach":true,"pasta":true},"country":"france"},{"id":"c","name":"george","food_prefs":{"tomatoes":true,"spinach":false,"pasta":false},"country":"argentina"}]

const result = lovesTomatoes(data)

console.log(result)

<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.28.0/ramda.min.js" integrity="sha512-t0vPcE8ynwIFovsylwUuLPIbdhDj6fav2prN9fEu/VYBupsmrmk9x43Hvnt+Mgn2h5YPSJOk7PMo9zIeGedD1A==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

Filtering using a pipe:

Using R.pipe. I wouldn't go this way for a simple filter by nested properties, but you can use a Schwartzian transform. The idea is to create a new array if pairs [value of tomatoes, original object], filter by the value of tomatoes, and then extract the original object:

const { pipe, map, applySpec, path, identity, filter, last, head } = R

const lovesTomatoes = pipe(
  map(applySpec([path(['food_prefs', 'tomatoes']), identity])), // create an array of [value of tomatoes, original object] 
  filter(head), // filter by the value of the tomatoes
  map(last) // extract the original object
)

const data = [{"id":"a","name":"fred","food_prefs":{"tomatoes":true,"spinach":true,"pasta":false},"country":"singapore"},{"id":"b","name":"alexandra","food_prefs":{"tomatoes":false,"spinach":true,"pasta":true},"country":"france"},{"id":"c","name":"george","food_prefs":{"tomatoes":true,"spinach":false,"pasta":false},"country":"argentina"}]

const result = lovesTomatoes(data)

console.log(result)

<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.28.0/ramda.min.js" integrity="sha512-t0vPcE8ynwIFovsylwUuLPIbdhDj6fav2prN9fEu/VYBupsmrmk9x43Hvnt+Mgn2h5YPSJOk7PMo9zIeGedD1A==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

How to combine the 1st lovesTomatoes filtering function in a pipe:

However, if you just need the pipe to perform other operations on the filtered array, use the filter as one of the steps:

const { filter, path, identity, pipe, map, prop, uniq } = R

const lovesTomatoes = filter(path(['food_prefs', 'tomatoes']))

const countriesOfTomatoLovers = pipe(
  lovesTomatoes,
  map(prop('country')),
  uniq
)

const data = [{"id":"a","name":"fred","food_prefs":{"tomatoes":true,"spinach":true,"pasta":false},"country":"singapore"},{"id":"b","name":"alexandra","food_prefs":{"tomatoes":false,"spinach":true,"pasta":true},"country":"france"},{"id":"c","name":"george","food_prefs":{"tomatoes":true,"spinach":false,"pasta":false},"country":"argentina"}]

const result = countriesOfTomatoLovers(data)

console.log(result)

<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.28.0/ramda.min.js" integrity="sha512-t0vPcE8ynwIFovsylwUuLPIbdhDj6fav2prN9fEu/VYBupsmrmk9x43Hvnt+Mgn2h5YPSJOk7PMo9zIeGedD1A==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

edited Feb 2, 2022 at 17:44

answered Feb 2, 2022 at 14:55

Ori Drori

196k32 gold badges243 silver badges233 bronze badges

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10 Comments

Emman Over a year ago

Wonderful, thank you. Is there a way to use R.path() to navigate to food_prefs? In other words, if we wanted to stick to the R.pipe() syntax, how should we do it?

Ori Drori Over a year ago

Pipe is not relevant for this problem. I can add a solution that uses a pipe, but it would be much more complicated. And the question is why?

Emman Over a year ago

Because (for me at least) it's easier to understand the functional steps: (1) go to location x in the nested hierarchy, then (2) filter property y according to whether it has value z. So the pipe() makes it an explicit set of steps...

Chad S. Over a year ago

Jus wanted to point out that filter(pathSatisfies(identity, ['food_prefs', 'tomatoes'])) is equivalent to filter(R.path(['food_prefs', 'tomatoes'])). pathSatisfies is great, but not needed here..

Scott Sauyet Over a year ago

@Emman: if it's not yet clear, the point is that filter (path (['food_prefs', 'tomatoes']) is a step in a pipeline. It takes an array, and returns a new array containing the appropriate subset of the original one. If you want to do more, simply wrap this inside your pipeline, between whatever steps you find it appropriate.

|

ThanosDi · Accepted Answer · 2022-02-02 15:20:27Z

2

const myData = [
    {
        "id": "a",
        "name": "fred",
        "food_prefs": {
            "tomatoes": true,
            "spinach": true,
            "pasta": false
        },
        "country": "singapore"
    },
    {
        "id": "b",
        "name": "alexandra",
        "food_prefs": {
            "tomatoes": false,
            "spinach": true,
            "pasta": true
        },
        "country": "france"
    },
    {
        "id": "c",
        "name": "george",
        "food_prefs": {
            "tomatoes": true,
            "spinach": false,
            "pasta": false
        },
        "country": "argentina"
    }
];

const lovesTomatoes = filter(pathOr(false, ['food_prefs','tomatoes']));

lovesTomatoes(myData);

Ramda REPL

answered Feb 2, 2022 at 15:20

ThanosDi

3383 silver badges11 bronze badges

2 Comments

Emman Over a year ago

Thank you. Would you mind advising why translating to the following pipe fails? const lovesTomatoes_pipe = R.pipe(R.pathOr(false, ["food_prefs"]), R.filter(R.prop("tomatoes"))) I'm trying to adopt the syntax of this answer but so far unsuccessfully.

ThanosDi Over a year ago

The first function of pipe R.pathOr(false, ["food_prefs"]) is not on a map so it will just return false and then the second function R.filter(R.prop("tomatoes")) will try to filter a boolean, not an array.

Hitmands · Accepted Answer · 2022-02-03 11:21:17Z

1

Ramda comes with a whole suite of predicates built-in already, one of them that I'd use here is pathEq.

I'd suggest to adopt a map and reduce kind of approach, whereas the match function is separated from the actual aggregation...

Collect your data point
Reduce it to the information you need

const tomatoLovers = R.filter(
  R.pathEq(['food_prefs', 'tomatoes'], true),
);

const avgAge = R.pipe(R.pluck('age'), R.mean);

const data = [{
    "id": "a",
    age: 16,
    "name": "fred",
    "food_prefs": {
      "tomatoes": true,
      "spinach": true,
      "pasta": false
    },
    "country": "singapore"
  },
  {
    "id": "b",
    age: 66,
    "name": "alexandra",
    "food_prefs": {
      "tomatoes": false,
      "spinach": true,
      "pasta": true
    },
    "country": "france"
  },
  {
    "id": "c",
    age: 44,
    "name": "george",
    "food_prefs": {
      "tomatoes": true,
      "spinach": false,
      "pasta": false
    },
    "country": "argentina"
  }
]

console.log(
  'Average age of tomato lovers is:',
  R.pipe(tomatoLovers, avgAge) (data),
);

console.log(
  'They are the tomato lovers',
  R.pipe(tomatoLovers, R.pluck('name')) (data),
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.28.0/ramda.js" integrity="sha512-ZZcBsXW4OcbCTfDlXbzGCamH1cANkg6EfZAN2ukOl7s5q8skbB+WndmAqFT8fuMzeuHkceqd5UbIDn7fcqJFgg==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

edited Feb 3, 2022 at 11:21

answered Feb 3, 2022 at 10:56

Hitmands

14.2k4 gold badges41 silver badges78 bronze badges

1 Comment

Emman Over a year ago

Thank you! I wonder about tomatoLovers(). Isn't filter(path(['food_prefs', 'tomatoes'])) more straightforward? Or does R.pathEq() have an advantage here?

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