Backend Application with MongoDB

With MongoDB each DDD Aggregate becomes a Collection Document.

With a Documental Database like MongoDB, mapping OneToMany and OneToOne relationships is trivial because they are represented as nested documents.

On the other hand ManyToOne and ManyToMany relationships needs a little more thought, and it greatly depends on the use case how you decide to denormalize the data.

OneToMany and OneToOne with Direct References

With ZenWave ZDL you map OneToOne and OneToMany relationships as direct references to @embedded entities:

@aggregate
@auditting
entity Customer {
  username String required unique /** username javadoc comment */
  email String required unique /** email javadoc comment */
  tags String[] /** tags javadoc comment */
  /**
   * addresses is a direct reference to an embedded entity
   */
  addresses Address[]
}

@embedded
entity Address {
    street String /** street javadoc comment */
    city String /** city javadoc comment */
    state String /** state javadoc comment */
    zip String /** zip javadoc comment */
    type AddressType /** address type is an enum */
}

OneToMany and OneToOne with Nested Objects

The following example is equivalent to the previous one, but it uses nested objects instead of direct references. This improves readability and expressiveness.

@aggregate
@auditting
entity Customer {
  username String required unique /** username javadoc comment */
  email String required unique /** email javadoc comment */
  tags String[] /** tags javadoc comment */
  /**
   * addresses is a nested entity
   */
  addresses Address[] {
    street String /** street javadoc comment */
    city String /** city javadoc comment */
    state String /** state javadoc comment */
    zip String /** zip javadoc comment */
    type AddressType /** address type is an enum */
  }
}

OneToMany and OneToOne with @DBRef and @DocumentedReference

@aggregate
@auditting
entity Customer {
  username String required unique /** username javadoc comment */
  email String required unique /** email javadoc comment */
  tags String[] /** tags javadoc comment */
  /**
   * addresses is mapped using @DocumentedReference
   */
  @ref addresses Address[]
}

@aggregate
@auditting
entity Address {
    street String /** street javadoc comment */
    city String /** city javadoc comment */
    state String /** state javadoc comment */
    zip String /** zip javadoc comment */
    type AddressType /** address type is an enum */
}