I have two different collections in my database, and at this point I need to link a document from first collection to a document form second collection with native NodeJs driver.
How can I do that? Should I hold the _id from first collection in second collection?
Yes, I would suggest storing the _id of the related document, manually, as you described. I've done that a lot, and it works well.
A more complex (and seemingly unnecessary in the case you describe), would be to use a DBRef.
Related
I am trying to query my Firestore collection (in Node.js for my flutter app), and to get the 10 documents which has the most objects in their subcolllection called Purchases (to get the best sellers).
Is it possible in Firestore? Or do I have to keep an int field outside of my subcollection to represent its length?
Thank you!
I thought this was answered recently, but can't find it right now, so...
Firestore queries (and other read operations) work on a single collection, or a group of collections with the same name. They don't consider any data in other (nested or otherwise) collections, nor can they query based on aggregates (such as the number of documents), unless those aggregates are stored in a document in the queried collection.
So the solution is indeed to keep a counter in a document in the collection you are querying against, and updating that counter with every add/delete to the subcollection.
Im using MongoDB for storing information for a nodeJS application and a doubt came to my mind, after finding that it is possible to use ObjectID to ref another document. As it is known, MongoDB is a no-SQL db, so there is no need for consistency whatsoever and information can be repeated.
So, lets say, I have a collection for users and one of their field values is 'friends', which is an array of this user friends (another users). What is the best practice, saving all the user info there (thus repeating the same thing over and over again throughout the DB) or saving only the ObjectID of the friendUser (makes way more sense to me, but it sounds kinda SQL mindset). I'm not really getting when should I use each of the options, so a professional opinion would be very appreciated.
To model relationships between connected data, you can reference a document or embed it in another document as a subdocument.
Referencing a document does not create a “real” relationship between these two documents as does with a relational database.
Referencing documents is also known as normalization. It is good for data consistency but creates more queries in your system.
Embedding documents is also known as denormalization.
The benefit of Embedding approach is getting all the data you need about a document and it’s sub-document(s) with a single query. Therefore, this approach is very fast. The drawback is that data may not stay as consistent in the database.
Important
If one document is to be used by many documents then better create a referenced doc.
i. Will Save Space.
ii. if any change required, we will have to update only the referenced doc
instead of updating many docs.
Create sub doc(embedded)
i. If another document is not dependent on the subdocument.
Source: https://vegibit.com/mongoose-relationships-tutorial/
Recommended reading:
MongoDB Applied Design Patterns by Rick Copeland
To Embed or Reference
I'm looking at the possibility of switching from Filemaker to a MERN app. I get the basic idea of NoSQL databases but its a very different way of thinking. I have recreated what I need in MySQL, but Mongo DB (or Firestore) interests me.
The app is used to record workouts of clients. Each person has many workouts, each workout has many items within it. I'm not sure the best way to do this in Mongo.
I'm between using a collection for all workouts, and each workout has an array of all the items (exercises) they did. Or have the items as a separate collection with a workout_ID to link them up.
Probably the most important thing I'm looking for here is the ability to see all the instances someone did an exercise. From my understanding, if I do it the first way, every single record will be pulled. But the second would only return the documents in that collection that would match exercise_ID and person_ID.
Which is the best approach for such a project?
The second option is the best way to store the data in your case. You can always separate the items in a different collection and store the item Ids in your main collection i.e workouts here.
If you are concerned about querying(joins) them, mongoose provide 'populate' to fetch data between multiple collections using ids as references.
Or you can also use aggregate queries to achieve the same.
hope this helps :)
I think the best option for me would be having an array of the items inside each workout, because that's basically the purpose of using no-SQL databases, to access faster to the data.
However there's a basic rule.. if you think the array of items of each workout it's gonna grow very fast and it will get so big.. then it's better to have a separate collection for the items. if it's not.. then keep those inside an array.
Hope this can be helpful to you.. greetings! :)
Mongoose doesn't appear to support Mongo DBRefs. Apparently they released "DBRef" support but it was actually just plain references (no ability to reference documents from different collections). I've finally managed to craft a schema that allows me to hold an array of ObjectID references and populate them, which is great for certain parts of my schema, but it would be extremely convenient if I could use proper DBRefs to create an array that lets me refer to documents from a number of collections.
Luckily(?) there's a module that can monkey patch DBRef support into mongoose: https://github.com/goulash1971/mongoose-dbref
Unluckily, I can't make any sense of the documents. The best I can tell is that there is no ability to use DBRefs in an array (there is a 'fetch' method to dereference, but it takes a single dbref); 'populate' doesn't seem to be patched to fill in DBRefs, and I can't tell how I'm supposed to assign a DBRef given a source document [collection.items.push(?????)].
From the internet, it appears that I can assign an object of the form { $id: document._id, $ref: 'Collection' } -- when logging the result, it appears to have "taken" as a DBRef data type, but I am unsure if this is correct since I cannot seem to do anything useful with it (turn the ref back into a document).
What I really want is a way to represent an ordered list of items from multiple collections; any solution to this is fine by me, but so far DBRefs are the best I've got. Help?
A DBRef (as explained in detail here) is a tuple containing the ObjectId, collection name, and possibly the database container name of a referenced object in another collection.
Internally in the MongoDB server these serve no purpose and are just data within a document. The point is for use in some drivers and ODM implementations to allow for some sort of automatic expansion by issuing additional queries to the server in order to have the data that is elsewhere appear to be an ordinary sub-document part of the referencing document. This can be automatic or a lazy load depending on the implementation, but is always done over the wire and processed on the client side. The server will do nothing to traverse or join this data.
Additionally, MongoDB collections are schemaless, so there is nothing as in the relational sense that says all documents in a collection have to have the same structure.
In the case of Mongoose, there are built in functions to do this sort of loading for you as a convenience, and while not strictly a DBRef and utilizing documents with a different schema in the same collection is the same means as storing the documents external to the referencing document.
It is important to consider the data access patterns of your application and not to simply opt for the same sort of relational design you are used to. Keeping in mind that you are only ever reading from one collection at a time, it is most desirable to get at the data you need in a single read or write, without multiple operations over the wire, which will slow things down considerably.
In short, you should always consider embedding sub-documents first, and then use external references any your best supported form only when you absolutely have to. Your application users will thank you in the end.
Is there a way to create and query a collection thats inside another collection...
Can any one direct me to literature that can teach me how to do this?
I've checked the mongodb
docs tutorial and i havent come across this scenario
No code example because i don't know where to start.
Here is what im trying to do.
Im modelling an online shop. So shop object details are in a collection holding many shops.
Now each shop has got products and each product its own unique details. I want to a void using references and rather append but from tutorial from mongodb docs its not quite clear how to do it.
Im fairly new to mongodb
Thanks.
There isn't any such thing as a collection inside another collection with mongo. A collection is a collection of documents.
There are a couple of ways to link documents to other documents, or collections of other data. The correct one to choose is up to you, your data, and your application.
Any document can point to another ObjectId from any field
You can have an array inside a document
Combine the two, and you can have an array of document ids
Mongo is a document store, not a relational database. My suggestion is don't try to use it as a relational database. If what you need is an RDBMS, use one.
Arrays:
http://docs.mongodb.org/manual/reference/operator/query-array/
http://docs.mongodb.org/manual/reference/operator/update-array/
ObjectId:
http://docs.mongodb.org/manual/core/document/