An Object/Document Mapping (ODM) framework for Node.js and MongodDB
Hydrate provides a means for developers to map Node.js classes to documents stored in a MongoDB database. Developers can work normally with objects and classes, and Hydrate takes care of the onerous details such as serializing classes to documents, validation, mapping of class inheritance, optimistic locking, fetching of references between database collections, change tracking, and managing of persistence through bulk operations.
Hydrate is inspired by other projects including JPA, Hibernate, and Doctrine.
TypeScript is a superset of JavaScript that includes type information and compiles to regular JavaScript. If you choose to use TypeScript in your projects, this type information can be used to create the mappings between your classes and the MongoDB documents, reducing duplicate work. However, TypeScript is not required and you can use Hydrate with plain JavaScript.
Decorators are a method of annotating classes and properties in JavaScript at design time. There is currently a proposal to include decorators as a standard part of JavaScript in ES7. In the meantime, several popular transpilers including Babel and TypeScript make decorators available for use now. Hydrate gives developers the option to leverages decorators as simple means to describe persistent classes.
Hydrate uses an approach to the persistence API similar to Hibernate ORM. Developers familiar with this approach should feel at home with Hydrate. Furthermore, Hydrate's query API is kept as similar as possible to the MongoDB native Node.js driver.
MongoDB bulk write operations are used to synchronize changes with the database, which can result in significant performance gains.
Hydrate requires a minimum of MongoDB 2.6 and Node 4.0. Once these dependencies are installed, Hydrate can be installed using npm:
$ npm install hydrate-mongodb --saveFor brevity, the example here is only given in TypeScript. JavaScript examples coming soon.
In this example we'll model a task list. We create a file, model.ts, defining entities Task and Person. We also
define an enumeration used to indicate the status of a task on the task list.
model.ts:
import {Entity, Field} from "hydrate-mongodb";
export enum TaskStatus {
Pending,
Completed,
Archived
}
@Entity()
export class Person {
@Field()
name: string;
constructor(name: string) {
this.name = name;
}
}
@Entity()
export class Task {
@Field()
text: string;
@Field()
status: TaskStatus;
@Field()
created: Date;
@Field()
assigned: Person;
constructor(text: string) {
this.created = new Date();
this.status = TaskStatus.Pending;
this.text = text;
}
archive(): boolean {
if(this.status == TaskStatus.Completed) {
this.status = TaskStatus.Archived;
return true;
}
return false;
}
}Once our model is defined, we need to tell Hydrate about it. We do this by adding the model to an AnnotationMappingProvider, then adding the mapping provider to the Configuration.
server.ts:
import {MongoClient} from "mongodb";
import {Configuration, AnnotationMappingProvider} from "hydrate-mongodb";
import * as model from "./model";
var config = new Configuration();
config.addMapping(new AnnotationMappingProvider(model));We use the standard MongoDB native driver to establish a connection to MongoDB. Once the connection is open, we create a SessionFactory using the MongoDB connection and the previously defined Configuration.
server.ts (con't):
MongoClient.connect('mongodb://localhost/mydatabase', (err, db) => {
if(err) throw err;
config.createSessionFactory(db, (err, sessionFactory) => {
...
});
});A Hydrate Session should not be confused with the web-server session. The Hydrate Session is analogous to JPA's EntityManager, and is responsible for managing the lifecycle of persistent entities.
Typically the SessionFactory is created once at server startup and then used to create a Session for each connection to the server. For example, using a Session in an Express route might look something like this:
app.get('/', function (req, res, next) {
var session = sessionFactory.createSession();
...
session.close(next);
}); Calling close on the Session persists any changes to the database and closes the Session. Call flush instead to persist any changes without closing the Session.
In order to create a new Task we instantiate the task and then add it to the
Session by calling
save.
var task = new Task("Take out the trash.");
session.save(task);To find a task by identifier we use find.
session.find(Task, id, (err, task) => {
...
});To find all tasks that have not yet been completed, we can use the query method.
session.query(Task).findAll({ status: TaskStatus.Pending }, (err, tasks) => {
...
});Below is an example of finding all tasks assigned to a specific person. Note that even though person is an instance
of the Person entity which is serialized as an ObjectId in the task collection, there is no need to pass the
identifier of the person directly to the query.
session.find(Person, personId, (err, person) => {
...
session.query(Task).findAll({ assigned: person }, (err, tasks) => {
...
});
}); Hydrate provides a mechanism to retrieve references between persistent entities. We do this using fetch. Note that fetch uses the same dot notation that MongoDB uses for queries.
For example, say we wanted to fetch the Person that a Task is assigned to.
session.fetch(task, "assigned", (err) => {
console.log(task.assigned.name); // prints the name of the Person
});The fetch method can be used in conjunction with queries as well.
session.find(Task, id).fetch("assigned", (err, task) => {
...
});All queries can use a Promise for the query result by calling asPromise.
session.find(Task, id).asPromise().then((task) => {
...
});session.query(Task).findAll({ assigned: person }).asPromise().then((tasks) => {
...
});Queries that return multiple entities may return an Observable for the query by calling asObservable.
session.query(Task).findAll({ assigned: person }).asObservable().subscribe((task) => {
...
});In TypeScript, the emitDecoratorMetadata and experimentalDecorators options must be enabled on the compiler.
EntitiesCollectionsFieldsIdentityEmbeddablesTypesInheritanceMapped SuperclassDiscriminatorsFetching
Entities are classes that map to a document in a MongoDB collection.
@Entity()
export class Person {
@Field()
name: string;
constructor(name: string) {
this.name = name;
}
}- The entity must be a class
- The entity must be decorated with the Entity decorator
- The entity is not required to have a parameterless constructor, which is different than JPA and Hibernate. This allows for entities to enforce required parameters for construction. When an entity is deserialized from the database, the constructor is not called. This means the internal state of an entity must fully represented by it's serialized fields.
- An identifier is assigned to the entity when it is saved.
- If the Immutable decorator is specified on an Entity, the entity is excluded from dirty checking.
If a name for the collection is not given, an entity is mapped to a collection in MongoDB based on the name of the class. The collectionNamingStrategy in the Configuration is used to determine the name of the collection. The default naming strategy is CamelCase. Alternatively, a name for the collection can be specified using the Collection decorator.
@Entity()
@Collection("people")
export class Person {
@Field()
name: string;
constructor(name: string) {
this.name = name;
}
}Fields are mapped on an opt-in basis. Only fields that are decorated are mapped. The name for the field in the document can optionally be specified using the Field decorator.
@Entity()
export class User {
@Field("u")
username: string;
}If the name for the field is not specified, the fieldNamingStrategy on the Configuration is used to determine the name of the field. The default naming strategy is CamelCase.
The identityGenerator on the Configuration is used to generate an identifier for an entity. The default identity generator is the ObjectIdGenerator. This is the only generator that ships with Hydrate. Composite identifiers are not supported. Natural identifiers are not supported.
The identifier is exposed on an entity as a string through the id property and in it's native format, typically ObjectID,
on the _id property. This is the default behavior and cannot be disabled. No decorator is required.
@Entity()
export class User {
_id: ObjectID;
id: string;
@Field()
username: string;
}If you do not want to use one or more of the identity properties, you can leave them off your class definition.
@Entity()
export class User {
id: string;
@Field()
username: string;
}Embeddables are classes that map to nested subdocuments within entities, arrays, or other embeddables.
@Embeddable()
export class HumanName {
@Field()
last: string;
@Field()
first: string;
@Field()
name: string;
constructor(last: string, first?: string) {
this.last = last;
this.first = first;
this.name = last;
if(first) {
this.name += ", " + first;
}
}
}
@Entity()
export class Person {
@Field()
name: HumanName;
constructor(name: HumanName) {
this.name = name;
}
}- The embeddable must be a class
- The embeddable must be decorated with the Embeddable decorator
- Like an entity, an embeddable is not required to have a parameterless constructor. When an embeddable is deserialized from the database, the constructor is not called. This means the internal state of an embeddable must fully represented by it's serialized fields.
- If the Immutable decorator is specified on an Embeddable class, the original document for the Embeddable is cached and used for serialization.
When using TypeScript, the type of a field is automatically provided. The following types are supported:
- Number
- String
- Boolean
- Date
- RegExp
- Buffer
- Array
- Enum
- Embeddables
- Entities
Type Decorator
When a property is an embeddable or a reference to an entity, sometimes the type of the property cannot be determined
because of circular references of import statements. In this case the
Type decorator should be used with the name
of the type.
@Entity()
export class Person {
@Type("HumanName")
name: HumanName;
constructor(name: HumanName) {
this.name = name;
}
}Arrays
TypeScript does not provide the type of an array element, so the type of the array element must be indicate with the ElementType decorator.
@Entity()
export class Organization {
@ElementType(Address)
addresses: Address[];
}This is true for primitive types as well.
@Entity()
export class Person {
@ElementType(String)
aliases: string[];
}Enums
By default enums are serialized as numbers. Use the Enumerated decorator to serialize enums as strings.
export enum TaskStatus {
Pending,
Completed,
Archived
}
@Entity()
export class Task {
@Field()
text: string;
@Enumerated(TaskStatus)
status: TaskStatus;
}Standard prototypical inheritance is supported for both entities and embeddables.
@Entity()
class Party {
...
}
@Entity()
class Person extends Party {
...
}
@Entity()
class Organization extends Party {
...
}All entities within an inheritance hierarchy are stored in the same collection. If the Collection decorator is used, it is only valid on the root of an inheritance hierarchy.
Entities stored in separate collections may share a common superclass that is not mapped to a collection. In the example,
below Patient (stored in patient collection) and Document (stored in document collection) share a common
superclass Asset that defines the field owner.
class Asset {
@Field()
owner: Organization;
constructor(owner: Organization) {
this.owner = owner;
}
}
@Entity()
class Patient extends Asset {
...
}
@Entity()
class Document extends Asset {
...
}If Asset was decorated with Entity then Patient
and Document would instead both be stored in a collection called asset.
If an inheritance hierarchy is defined, a discriminator field is added to the serialized document to indicate the type when deserializing the entity or embeddable. By default, the discriminatorField on the Configuration is used to determine the name of the field to store the discriminator. Optionally, the discriminator field can be specified on the root of an inheritance hierarchy using the DiscriminatorField decorator.
@Entity()
@DiscriminatorField("type")
class Party {
...
}The class discriminator can be specified using the DiscriminatorValue decorator.
@Entity()
@DiscriminatorValue("P")
class Person extends Party {
...
}
@Entity()
@DiscriminatorValue("O")
class Organization extends Party {
...
}If the discriminator value is not explicitly specified for a class, it is determined using the discriminatorNamingStrategy on the Configuration. By default, the name of the class is used.
By default entity references are not loaded and must be fetched using Session#fetch or similar. If a FetchType of Eager is specified on an entity reference then that reference is automatically fetched when the entity is loaded.
- This works on entity reference in Embeddable objects as well.
- It is generally preferable to fetch references as needed.
- A FetchType of Eager on a property that is not an entity reference has no effect.
@Entity()
export class Task {
@Fetch(FetchType.Eager)
owner: Person;
}When an entity is loaded, all fields for that entity are retrieved from the database. Specifying a FetchType of Lazy for a field causes that field to not be retrieved from the database when the entity is loaded. The field is only loaded by calling Session#fetch and indicating which field to load.
- Useful for properties that contain large amounts of data, such as images, that are not always needed.
- A FetchType of Lazy on a property in an Embeddable objects is ignored. All properties in an embeddable object are always loaded from the database.
- It is generally not advisable to use a FetchType of Lazy on a property that is an entity reference.
@Entity()
export class Person {
@Fetch(FetchType.Lazy)
image: Buffer;
}