Using MongoDB/mongoose/typegoose and typesCript, I have some user , car and phone models where I need to have as an attribute of user, an array with the products (car and phone) that are related to it. You could use populate but you would need to make one for each product. It is not clear to me how inheritance is handled in MongoDB so that my project is scalable.
I'm using Sequelize as my ORM, and just wondering what the point of having a model is.
It looks like the main thing that matters, is the table definitions in your migrations, and models are just a static snapshot of what your tables look like. When you perform a migration, nothing changes in your models. It doesn't get updated, nor created/deleted based on your migration.
You have to manually keep your models up to date it looks like.
So is there any point in having models, or making the effort to keep them updated?
The models are the definition of your database schema so that it can map into the ORM that Sequelize provides. For me this is the most important feature of Sequelize, not the migrations.
Migrations are used for changing the database schema.
Models are used to map the database schema to your code.
Using Models gives you lots of built in helper methods, associations let you build references between tables to generate complex JOINs, etc.
When looking at tutorials there is often a delineation between a schema and a model, particularly when dealing with mongoose/mongodb.
This makes porting over to postgresql somewhat confusing, as 'models' don't seem to exist under that system. What is the difference the two approaches?
For example, what would be a postgres/sql ORM equivalent of this line?
(mongoose and express.js):
var userSchema = schema.define('local', {
username: String,
password: String,
});
module.exports = mongoose.model('User', userSchema);
In mongoose, a schema represents the structure of a particular document, either completely or just a portion of the document. It's a way to express expected properties and values as well as constraints and indexes. A model defines a programming interface for interacting with the database (read, insert, update, etc). So a schema answers "what will the data in this collection look like?" and a model provides functionality like "Are there any records matching this query?" or "Add a new document to the collection".
In straight RDBMS, the schema is implemented by DDL statements (create table, alter table, etc), whereas there's no direct concept of a model, just SQL statements that can do highly flexible queries (select statements) as well as basic insert, update, delete operations.
Another way to think of it is the nature of SQL allows you to define a "model" for each query by selecting only particular fields as well as joining records from related tables together.
In other ORM systems like Ruby on Rails, the schema is defined via ActiveRecord mechanisms and the model is the extra methods your Model subclass adds that define additional business logic.
A schema is fundamentally describing the data construct of a
document (in MongoDB collection). This schema defines the name of each item of data, and the type of data, whether it is a string, number, date, Boolean, and so on.
A model is a compiled version of the schema. One instance of the model will map to one document in the database.
It is the model that handles the reading, creating, updating, and deleting of documents.
A document in a Mongoose collection is a single instance of a model. So it makes sense that if we're going to work with our data then it will be through the model.
A single instance of a model (like a User instance in var User = mongoose.model('User', userSchema);) maps directly to a single document in the database.
With this 1:1 relationship, it is the model that handles all document interaction - creating, reading, saving, and deleting. This makes the model a very powerful tool.
Taken from "Mongoose for Application Development", by Simon Holmes, 2013
I imagine models as classes created from a schema (maybe I am mistaken).
MongoDB stores everything in BSON , which is a binary format. A simple Hello World BSON document might look like this internally:
\x16\x00\x00\x00\x02hello\x00\x06\x00\x00\x00world\x00\x00. A computer can deal with all that mumbo-jumbo, but that's hard to read for humans. We want something we can easily understand, which is why developers have created the concept of a database model. A model is a representation of a database record as a nice object in your programming language of choice. In this case, our models will be JavaScript objects. Models can serve as simple objects that store database values, but they often have things like data validation, extra methods, and more. As you’ll see, Mongoose has a lot
of those features.
Taken from "Express in Action", by Evan Hahn, 2016
In Short:
A Mongoose model is a wrapper on the Mongoose schema. A Mongoose schema defines the structure of the document, default values, validators, etc., whereas a Mongoose model provides an interface to the database for creating, querying, updating, deleting records, etc.
Reference: Introduction to Mongoose for MongoDB - FCC
If I define a nested model or a relation in a mongoose schema, does it have a mechanism to autoload everything when re-hydrating instances, or do I simply have to ensure I've always loaded the appropriate models?
Nested, embedded model docs are auto-loaded, but if it's an ObjectId based relation, then you need to use Mongoose's populate support to explicitly load the related docs.
Mongoose performs all these actions based on the schemas and models that your code defines, typically during your app's startup.
Looking at the Mongoose ODM docs, it doesn't really say much about what are ObjectId's and how they can be used. I think its something like foreign keys in MongoDB?
If so, Embedded Documents seem to achieve the same purpose, when do I use which?
It would be very worthwhile to read the MongoDB documentation or a quick MongoDB intro such as The Little MongoDB Book (it's free) for some background on MongoDB concepts.
To answer your question:
An ObjectID is a unique 12-byte identifier which can be generated by MongoDB as the primary key (_id) for a collection. There is a specification for the ObjectID.
A DBRef (database reference) is an ObjectID referencing an object in another collection. A DBRef does require require another query to fetch the related object, and is a convention supported by the client drivers rather than MongoDB server. The Mongoid equivalent is called referenced relations.
Embedded documents are nested arrays or subdocuments within a document. In Mongoid these are embedded relations.
The approach to data modelling and schema design in MongoDB is very different from relational databases. There are (intentionally) no joins or foreign keys, but the document-oriented approach allows large amounts of related data to be stored and fetched in a single document. Depending on how you plan to query and update your data, embedding or linking may be a more suitable choice. The schema design page on the MongoDB wiki has some helpful tips to get you started.