CouchDB display distinct values - couchdb

Question

I have a document like below,
{
"id": "7d9fdc2f4846544d62da3421bf011b31",
"al": [
{ "id16": "0x1d42",
"pos": {
"x": 10.32,
"y": 11.13,
"z": 1.22
},
"resultTime": "2020-06-01T20:45:34.976Z"
},
{ "id16": "0x1342",
"pos": {
"x": 0.32,
"y": 1.13,
"z": 13.22
},
"resultTime": "2021-06-01T20:45:34.976Z"
}
.
.
.
],
"Timestamp": 272179,
"Oid": "Onion1",
}
and Design document is like below
{
"id": "_design/GetALwithAnchorID",
"key": "_design/GetALwithAnchorID",
"value": {
"rev": "32-6db6c4e105336d47a6c8e7e8458ee345"
},
"doc": {
"_id": "_design/GetALwithAnchorID",
"_rev": "32-6db6c4e105336d47a6c8e7e8458ee345",
"views": {
"GetALwithAnchorID": {
"map": "function (doc) {\n\n for (var i=0; i<doc.al.length; i++) { \n emit(doc.al[i].id16, doc.al[i].pos);\n }\n \n}\n\n",
"reduce": "_approx_count_distinct"
}
},
"language": "javascript"
}
}
when I query the view like
http://127.0.0.1:5984/rtls/_design/GetALwithAnchorID/_view/GetALwithAnchorID?group_level=1&key=%220x1d42%22
I get the results as below
{"rows":[
{"key":"0x1d42","value":1}
]}
But I want distinct values of id16 and pos of id16. and to sort these distinct values by time and display the values of pos instead of "value":1 when Iquery?
thank you in advance.

Answer 1

OK so not quite the same as this similar answer. Anyone coming across this Q/A, I recommend reading over that answer.
Consider the following emit for your given doc structure:
doc.al.forEach(e => emit(
[e.pos.x, e.pos.y, e.pos.z, e.resultTime], // key
[e.id16, e.pos, e.resultTime]) // value
));
The emit's complex key visualized in the index (loosely not verbatim):
[-3,-2,-1,"2017-10-28T22:56:58.852Z"]
[-3,-2,-1,"2019-01-23T03:33:20.958Z"] **
. . .
[0,0,0,"2016-05-27T01:38:36.305Z"]
[0,0,0,"2016-12-27T05:17:02.255Z"] **
. . .
[1,2,3,"2016-11-14T17:31:59.468Z"]
[1,2,3,"2017-07-17T07:52:38.180Z"] **
Where each ** the last item in the pos group and significantly the most recent resultTime. All due to CouchDB's collation.
Working with CouchDB demands understanding the B-tree, and it's documentation has a great rundown of it in its Reduce/Rereduce documentation.
Now consider this reduce function:
function(keys,values,rereduce) {
return values[0];
}
It doesn't look terribly impressive, but further consider calling the view with these parameters:
{
reduce: true,
group_level: 1,
descending: true
}
By reversing the order of the index scan with descending the reduce function is guaranteed to return the most recent row with respect to resultTime of any given pos group.
Here's a simple demo using pouchDB. It generates 6 documents with random resultTime's and randomly selects pos from a pool of 3. Have a look at the design doc.
async function showReduceDocs(view) {
let result = await db.query(view, {
reduce: true,
group_level: 1,
descending: true
});
// show
debugger;
gel('view_reduce').innerText = result.rows.map(row => `${JSON.stringify(row.value)}`.split(',').join(', ')).join('\n');
return result;
}
async function showViewDocs(view) {
let result = await db.query(view, {
reduce: false,
include_docs: false
});
//show
gel('view_docs').innerText = result.rows.map(row => JSON.stringify(row.key))
.join('\n');
}
function getDocsToInstall(count) {
// design document
const ddoc = {
"_id": "_design/SO-66231293",
"views": {
"id16": {
"map": `function (doc) {
doc.al.forEach((e) => emit([e.pos.x, e.pos.y, e.pos.z, e.resultTime],[e.id16, e.pos, e.resultTime]));
}`,
"reduce": `function(keys,values,rereduce) {
return values[0];
}`
}
}
};
// create a set of random documents.
let docs = new Array(count);
let docId = 65;
const posSeed = [{
x: 0,
y: 0,
z: 0
},
{
x: 1,
y: 2,
z: 3
},
{
x: -3,
y: -2,
z: -1
}
];
const dateSeed = [new Date(2000, 0, 1), new Date(), 0, 24];
while (count--) {
let n = 6;
let doc = {
_id: String.fromCharCode(docId++),
al: new Array(n)
};
while (n-- > 0) {
doc.al[n] = {
"id16": "0x000" + n,
"pos": posSeed[Math.floor(Math.random() * 100) % 3],
"resultTime": randomDate(...dateSeed).toISOString()
};
}
docs[count] = doc;
}
docs.push(ddoc);
return docs;
}
const db = new PouchDB('SO-66231293', {
adapter: 'memory'
});
(async() => {
// install docs and show view in various forms.
await db.bulkDocs(getDocsToInstall(6));
gel('content').classList.remove('hide')
showReduceDocs('SO-66231293/id16');
showViewDocs('SO-66231293/id16');
})();
const gel = id => document.getElementById(id);
/*
https://stackoverflow.com/questions/31378526/generate-random-date-between-two-dates-and-times-in-javascript/31379050#31379050
*/
function randomDate(start, end, startHour, endHour) {
var date = new Date(+start + Math.random() * (end - start));
var hour = startHour + Math.random() * (endHour - startHour) | 0;
date.setHours(hour);
return date;
}
<script src="https://cdn.jsdelivr.net/npm/pouchdb#7.1.1/dist/pouchdb.min.js"></script>
<script src="https://github.com/pouchdb/pouchdb/releases/download/7.1.1/pouchdb.memory.min.js"></script>
<div id='content' class='hide'>
<div>View: reduce</div>
<pre id='view_reduce'></pre>
<hr/>
<div>View: complex key</div>
<pre id='view_docs'></pre>
</div>
Edit
Amended the demo snippet according to OP's comments.