How to query documents from a sub-collection, based on a main collection's field and a sub-collection's fields in cloud firestore? - firebase

Question

I am new to firebase cloud firestore and need help understanding it a bit.
I am working on a project of the following structure.
Each "Restaurant" document contains its own "products" subcollection.
Here, I wanted to run a query to get all the products by different
restaurants that contain the tag "coffee" and are in a specific
Pincode "234144".
I tried Collection group queries by adding Pincode to each product owned by a particular restaurant but changing a Pincode would cost a lot, as all products would have to be edited, I guess.
Is there any efficient way of doing it or is it not possible in this
database in an efficient way?
Please let me know what do you think... Thank you.

Answer 1

If I understand correctly, you want to retrieve all products with a certain tag and pincode (I suppose this is similar to a postal zipcode?). There really are only two alternatives that I can think of:
Collection group query
As you mention, you can store both tag and pincode in product documents. Then perform a single collection group query along these lines (pardon the javascript, but I am not familiar with Dart, it should be very similar):
var products = await firestore.collectionGroup('products')
.where('tag', '==', 'coffee')
.where('pincode', '==', '234144').get();
As you have noted, with this solution you need to keep pincode in each product This piece of data is duplicated and it is normal to feel that it should be avoided because it is wasteful and dangerous (can go out of sync), but this is the way to go! It is called denormalization. This is well explained in this video by Todd Kerpelman.
You can then create a Cloud Function triggered by restaurant update to keep pincode in products in sync with the corresponding pincode in restaurants
Query restaurants then products
To keep the pincode in restaurants only, you have to do your query in two steps: first filter restaurants in the certain pincode, then filter products by tag:
// 1 - retrieve restaurants in specific pincode
var restaurants = await firestore.collection('restaurants').where('pincode', '==', '234144').get();
// 2 - For each retrieved restaurant, retrieve all products matching the tag
var products = [];
for(let i = 0; i < restaurants.docs.length; ++i) {
var p = await restaurants.docs[i].collection("products").where("tag", "==", "coffee");
products.push(p);
}
With this method, no need to duplicate pincode in each product, however your queries are less optimal, because you load potentially useless restaurants that do not serve coffee in your pincode.

Answer 2

I have a similar data structure, and Louis Coulet's answer helped me a lot. However, I ran into an issue, as my node/firebase environment complained that restaurants.docs[i].collection was not a function. Instead, I had to use restaurants.docs[i].ref.collection.
So, in my case, here's the code that wound up working (using the original poster's "restaurants/products" data model as an example):
// 1 - retrieve restaurants in specific pincode
const restaurants = await firestore.collection('restaurants').where('pincode', '==', '234144').get();
// 2 - For each retrieved restaurant, retrieve all products matching the tag 'coffee'
const products = [];
for (let i = 0; i < restaurants.docs.length; i++) {
await restaurants.docs[i].ref
.collection('products')
.where('tag', '==', 'coffee')
.get()
.then(s => Promise.all(s.docs.map(d => {
products.push(d);
})));
}
I have found that I need to return the sub-collection contents via some kind of promise, otherwise, after I try to get the data in the variable -- products, in this case -- it just shows up as undefined.
After I populate the products variable, I am then able to read its contents like this:
products.forEach(p => {
console.log(p.data().name);
console.log(p.data().tag);
}
I hope someone finds this useful, and many, MANY thanks to Louis Coulet for sending me/us down the correct path with this sub-collection headache :-)