rxjs - searching in observables - search

Question

I am trying to implement a simple search scheme to search for values in one observable from another observable. The buildLookup function below builds a lookup table using values from an observable:
// Build lookup table from an observable.
// Returns a promise
function buildLookup(obs, keyName, valName) {
const map = new Map();
obs.subscribe((obj) => map.set(obj[keyName], obj[valName]));
// use concat to force wait until `obs` is complete
return obs.concat(Observable.from([map])).toPromise();
}
Then I have another function this uses the result of this function (a promise):
// Lookup in a previously built lookup table.
function lookup(source, prom, keyName, fieldName) {
return source.map((obj) => {
const prom2 = prom.then((map) => {
return lodash.assign({}, obj, { [fieldName]: map.get(String(obj[keyName])) });
});
return Observable.fromPromise(prom2);
})
.flatMap((x) => x);
}
For some reason, this implementation does not work, and every other lookup seems to fail. Could you someone guide me on:
what is wrong with this code, and
whether there is a better way to implement something like this?
Thanks a bunch in advance for your help!
I am attaching my test code below:
"use strict";
const lodash = require("lodash");
const rxjs = require("rxjs");
const chai = require("chai");
const Observable = rxjs.Observable;
const assert = chai.assert;
const assign = lodash.assign;
describe("search", () => {
it("simple search", (done) => {
let nextId = 1, nextId2 = 1;
const sourceObs = Observable.interval(5).take(5).map((i) => {
const id = nextId++;
return { id: `${id}` };
});
const searchableObs = Observable.interval(5).take(5).map((i) => {
const id = nextId2++;
return Observable.from([
{ id: `${id}`, code: "square", val: id * id },
]);
}).flatMap((x) => x);
const results = [];
const verifyNext = (x) => {
assert.isDefined(x);
results.push(x);
};
const verifyErr = (err) => done(err);
const verifyComplete = () => {
assert.equal(results.length, 5);
try {
results.forEach((r) => {
console.log(r);
// assert.equal(r.val, r.id*r.id); <== *** fails ***
});
} catch (err) {
done(err);
}
done();
};
// main
const lookupTbl = buildLookup(searchableObs, "id", "val"); // promise that returns a map
lookup(sourceObs, lookupTbl, "id", "val")
.subscribe(verifyNext, verifyErr, verifyComplete)
;
});
});
// output
// { id: '1', val: 1 }
// { id: '2', val: undefined }
// { id: '3', val: 9 }
// { id: '4', val: undefined }
// { id: '5', val: 25 }

Answer 1

So, a bunch of things to address here.
The main issue is that you are doing side-effects in your sourceObs and searchableObs observables, and it is not published, so the side-effects happen multiple times because you subscribe multiple times, giving you a wrong map entirely. For instance, I get maps like:
{"1" => 1, "4" => 16, "7" => 49, "12" => 144}
But you are doing something so trivial that you should really not use mutable variables.
To solve this, here is how you can create the proper observables:
const sourceObs = Rx.Observable.range(1, 5).map(i => ({ id: `${i}` }));
const searchableObs = Rx.Observable.range(1, 5).map(i =>
({ id: `${i}`, code: "square", val: i * i })
);
There is no reason to use a variable since range returns the numbers 1, 2, ...
And your use of o.map(_ => Rx.Observable.from(...)).concatMap(e => e) is really just the same as o...
While I'm here, this is a simplified version of your correct but clumsy functions:
// so buildLookup just needs to return a map once it's finished populating it
function buildLookup(obs, keyName, valName) {
// following your style here, though this could be done using `scan`
const map = new Map();
obs.subscribe((obj) => map.set(obj[keyName], obj[valName]));
// instead of your promise, I just wait for `obs` to complete and return `map` as an observable element
return obs.ignoreElements().concat(Rx.Observable.of(map));
}
// and lookup just needs to wait for the map, and then populate fields in the object
function lookup(source, prom, keyName, fieldName) {
return prom
.concatMap(map => source.map(obj => ({ obj: obj, map: map })))
.map(({ obj, map }) => lodash.assign({}, obj, { [fieldName]: map.get(String(obj[keyName])) }))
;
}
This should work for you.