How to summarize an array with group and rollup from d3-array?

Question

I'm trying to use d3-array to generate two summaries of an array of objects:

• What actions did each teacher perform?
• What posts did each teacher edit?

This is my current approach:

const data = [
  { post_id: 47469, action: "reply", teacher_username: "John" },
  { post_id: 47469, action: "edit", teacher_username: "John" },
  { post_id: 47468, action: "reply", teacher_username: "John" },
  { post_id: 47465, action: "reply", teacher_username: "Mary" },
  { post_id: 47465, action: "edit", teacher_username: "Mary" },
  { post_id: 47467, action: "edit", teacher_username: "Mary" },
  { post_id: 46638, action: "reply", teacher_username: "Paul" },
];

const teacherSummary = [
  ...d3.rollup(
    data,
    (x) => x.length,
    (d) => d.teacher_username,
    (d) => d.action
  ),
]
  .map((x) => {
    return {
      teacher_username: x[0],
      num_edits: x[1].get("edit") || 0,
      num_replies: x[1].get("reply") || 0,
    };
  })
  .sort((a, b) => d3.descending(a.num_edits, b.num_edits));
// [
//   { "teacher_username": "Mary", "num_edits": 2, "num_replies": 1 },
//   { "teacher_username": "John", "num_edits": 1, "num_replies": 2 },
//   { "teacher_username": "Paul", "num_edits": 0, "num_replies": 1 }
// ]

const postIdsByTeacher = d3.rollups(
  data.filter((x) => x.action === "edit"),
  (v) => [...new Set(v.map((d) => d.post_id))].join(", "), // Set() is used to get rid of duplicate post_ids
  (d) => d.teacher_username
);
// [
//  ["John","47469"],
//  ["Mary","47465, 47467"]
// ]

I'm flexible on the output format. What I'd like to optimize for is efficiency and clarity:

• Could I get both summaries in a single rollup call? Maybe by adding edited_post_ids to teacherSummary.
• It seems there should be a more elegant approach to replace the [...Map/Set] calls

Edit: Out of curiosity, I also tried this approach using alasql. Except for the nulls in edited_post_ids, it almost works.

sql = alasql(`
select
  teacher_username,
  count(case when action = 'reply' then 1 end) num_replies,
  count(case when action = 'edit' then 1 end) num_edits,
  array(case when action = 'edit' then post_id end) as edited_post_ids
from ?
group by teacher_username
`, [data])
// [ 
//   { teacher_username: "John", num_replies: 2, num_edits: 1, edited_post_ids: [null, 47469, null], }, 
//   { teacher_username: "Mary", num_replies: 1, num_edits: 2, edited_post_ids: [null, 47465, 47467], }, 
//   { teacher_username: "Paul", num_replies: 1, num_edits: 0, edited_post_ids: [null], },
// ];

Answer 1

The function signature for d3.rollup is:

d3.rollup(iterable, reduce, ...keys)

Which on the face of it says you can provide one operation in reduce e.g. counting or summation or some other operation - but just one.

For your output, you're looking for two different operations

• counting the replies and edits, and
• an array operation to get post_ids where action == "edit"

As soon as you made the choice to use (x) => x.length you've kind of cut off the opportunity to use a different reduce operation. Arguably d3.rollup isn't the function you need if you have multiple operations?

Still you can add edited_post_ids to teacherSummary by simply reaching back into the original data and applying a filter then map:

const data = [
  { post_id: 47469, action: "reply", teacher_username: "John" },
  { post_id: 47469, action: "edit", teacher_username: "John" },
  { post_id: 47468, action: "reply", teacher_username: "John" },
  { post_id: 47465, action: "reply", teacher_username: "Mary" },
  { post_id: 47465, action: "edit", teacher_username: "Mary" },
  { post_id: 47467, action: "edit", teacher_username: "Mary" },
  { post_id: 46638, action: "reply", teacher_username: "Paul" },
];

const teacherSummary = [...d3.rollup(
  data,
  v => v.length,
  d => d.teacher_username,
  d => d.action
)].map(d => {
  return {
    teacher_username: d[0],
    num_edits: d[1].get("edit") || 0,
    num_replies: d[1].get("reply") || 0,
    edited_post_ids: data
      .filter(x => x.action === "edit" & x.teacher_username == d[0])
      .map(x => x.post_id)
  }
});
  
console.log(teacherSummary);

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/6.0.0/d3.min.js"></script>

An alternative is to not use d3.rollup/d3.rollups and use d3.groups instead. The source for rollup and group are all calls to nest btw. You lose the counting that rollup did for you and have to implement it yourself. This example reads a bit more like the SQL example:

const data = [
  { post_id: 47469, action: "reply", teacher_username: "John" },
  { post_id: 47469, action: "edit", teacher_username: "John" },
  { post_id: 47468, action: "reply", teacher_username: "John" },
  { post_id: 47465, action: "reply", teacher_username: "Mary" },
  { post_id: 47465, action: "edit", teacher_username: "Mary" },
  { post_id: 47467, action: "edit", teacher_username: "Mary" },
  { post_id: 46638, action: "reply", teacher_username: "Paul" },
];

// compare with
// select
//   teacher_username,
//   count(case when action = 'reply' then 1 end) num_replies,
//   count(case when action = 'edit' then 1 end) num_edits,
//   array(case when action = 'edit' then post_id end) as 
// edited_post_ids
// from ?
// group by teacher_username

const teacherSummary = d3.groups(data, d => d.teacher_username)
  .map(k => {
    return {
      teacher_username: k[0],
      num_edits: k[1].filter(k2 => k2.action == "edit").length,
      num_replies: k[1].filter(k2 => k2.action == "reply").length,
      edited_post_ids: k[1].filter(k2 => k2.action == "edit").map(k3 => k3.post_id)
    }
  });
  
console.log(teacherSummary);

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/6.0.0/d3.min.js"></script>

As a side-note then, you can boil down postIdsByTeacher to below, and avoid new Set(etc) type stuff:

const data = [
  { post_id: 47469, action: "reply", teacher_username: "John" },
  { post_id: 47469, action: "edit", teacher_username: "John" },
  { post_id: 47468, action: "reply", teacher_username: "John" },
  { post_id: 47465, action: "reply", teacher_username: "Mary" },
  { post_id: 47465, action: "edit", teacher_username: "Mary" },
  { post_id: 47467, action: "edit", teacher_username: "Mary" },
  { post_id: 46638, action: "reply", teacher_username: "Paul" },
];

const postIdsByTeacher = d3.rollups(
  data.filter(d => d.action === "edit"),
  v => [].concat(v.map(k => k.post_id)),
  d => d.teacher_username
);

console.log(postIdsByTeacher);

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/6.0.0/d3.min.js"></script>

But my gut feel is the value of using d3.rollup is going to be when you want to do the standard summation and counting kind of stuff.

Answer 2

I ended up simplifying @Robin Mackenzie's last suggestion:

const uniq = require('lodash.uniq');
const teacherSummary = d3
  .groups(data, (d) => d.teacher_username)
  .map(([teacher_username, actions]) => {
    const edits = actions.filter((x) => x.action == "edit").map((x) => x.post_id);
    const replies = actions.filter((x) => x.action == "reply").map((x) => x.post_id);
    return {
      teacher_username,
      num_edits: edits.length,
      num_replies: replies.length,
      edited_post_ids: uniq(edits),
      replied_post_ids: uniq(replies),
    };
  })