How can I return child objects in a many-to-many relationship with Dropwizard/Hibernate/Swagger without recursion? - jackson

Question

I'm using Dropwizard and Swagger to create a REST API to manage access to various objects. Two of these objects are involved in a many-to-many relationship, for example:
public class Puppy implements Serializable {
private Long id;
private String name;
#ManyToMany(targetEntity = Trick.class)
#JoinTable(
name="puppies_tricks",
joinColumns=#JoinColumn(name="puppy_id"),
inverseJoinColumns=#JoinColumn(name="trick_id"))
private List<Trick> tricks;
#JsonProperty("tricks")
public List<Trick> getTricks() { return this.tricks; }
...
}
public class Trick implements Serializable {
private Long id;
private String name;
#ManyToMany(targetEntity = Puppy.class)
#JoinTable(
name="puppies_tricks",
joinColumns=#JoinColumn(name="trick_id"),
inverseJoinColumns=#JoinColumn(name="puppy_id"))
private List<Puppy> puppies;
#JsonProperty("puppies")
public List<Puppy> getPuppies() { return this.puppies; }
...
}
And suppose the actual data looks something like:
# Tricks:
[{ id: 1, name: 'Roll over' },
{ id: 2, name: 'Play dead' },
{ id: 3, name: 'Steal second' }]
# Puppies:
[{ id: 1, name: 'Flopsy' },
{ id: 2, name: 'Mopsy' },
{ id: 3, name: 'Cottontail' }]
# Puppies_Tricks
[{ puppy_id: 1, trick_id: 1 },
{ puppy_id: 1, trick_id: 2 },
{ puppy_id: 2, trick_id: 2 },
{ puppy_id: 2, trick_id: 3 }]
So when I GET /puppy/1 I want to get a structure like:
{
"id": 1,
"name": "Flopsy",
"tricks": [
{ "id": 1, "name": "Roll over"},
{ "id": 2, "name": "Play dead" } ]
}
but what I'm actually getting is a nested array:
{
"id": 1,
"name": "Flopsy",
"tricks": [
{ "id": 1, "name": "Roll over", "puppies": [ { "id": 1, "name": "Flopsy" }, ...},
{ "id": 2, "name": "Play dead", "puppies": [...] } ]
}
Is there some way to tell Jackson/Dropwizard to stop descending through tricks to when getting puppies, and vice versa?
Dropwizard 1.0.6, Java 1.8.

Answer 1

Jackson has support of object-graph serialization through the #JsonManagedReference #JsonBackReference annotations. See the documentation here. You can find a simple example here.
You should really read more carefully the documentation of the tools you're using.
There is already an answer that summarizes what approaches are available.

Answer 2

#JsonIgnore
#JsonProperty("puppies")
public List<Puppy> getPuppies() { return this.puppies; }
will stop infinite loop

Answer 3

My solution here was to create a separate PuppyCore class that implements serialization but isn't persisted. This includes basic Puppy info (id, name). Similarly a TrickCore has id and name only.
Then Puppy includes Set tricks, with this getter:
public Set<TrickCore> getTricks();
which returns a list of TrickCore objects – these don't have the Puppy list, and so halts the recursion. A similar pattern on Trick returns Set<PuppyCore> for getPuppies().