How to create a new entity with association - ajax

Question

Let's say i have a form on the frontend with usual fields and dropdowns.
In those dropdowns user is able to select an option, and each option is linked to an entity in Spring data JPA;
Dropdowns contain some label and a link to corresponding entity as a value.
This value is then passed in a POST-request to a PagingAndSorting repository of an entity which we wish to create.
Let's say it's a user with username and he must be associated with one of the offices (Also an entity):
#Data
#Builder
#Entity
#NoArgsConstructor
#AllArgsConstructor
#Table(name="users")
public class User{
#Id
#Coluemn(name="USER_ID")
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long userId;
#Column(name="USER_NAME", nullable=false)
private String userName;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name="OFFICE_ID", **nullable=false**)
private Office office;
}
My first guess would be:
Sending POST-request to http://localhost:8080/api/users/
contentType:'application/json'
{"userName":"Anton","office":"http://localhost:8080/api/offices/1"}
But it throws an exception
{
"cause": {
"cause": null,
"message": "Cannot construct instance of `test.domain.Office` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('http://localhost:8080/api/offices/1')\n at [Source: (org.apache.catalina.connector.CoyoteInputStream); line: 1, column: 160] (through reference chain: test.domain.User[\"office\"])"
},
"message": "JSON parse error: Cannot construct instance of `test.domain.Office` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('http://localhost:8080/api/offices/1'); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `test.domain.Office` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('http://localhost:8080/api/office/1')\n at [Source: (org.apache.catalina.connector.CoyoteInputStream); line: 1, column: 160] (through reference chain: test.domain.User[\"office\"])"
}
What am i doing wrong?

Answer 1

You are sending a URL resource as a string in place of a JSON object and expecting some magic to happen between Spring and jackson to look up the value. Naturally this is not what is happening and Jackson is attempting to bind the string value of the URL to the Office field. This fails of course because it does not know how to create an Office object from a string.
A possible solution is to make a distinction between your Entity Objects (those which represent your database tables) and DTO's (Data Transfer Objects) which in this cause represent your contract with your client. When doing this you could receive a User Object like so:
#Data
#Builder
#NoArgsConstructor
#AllArgsConstructor
public class User{
private Long userId;
private String userName;
private Long officeId;
}
Now you can simply send an office id instead of a URL and in your code use a Spring data repository to lookup the office object. After that you can then construct your Entity User object like the one you have shown above and persist it.

Answer 2

Turns out it was because i used Lombok, which generated it's own constructor.
To make it work i just needed to set #AllArgsConstructor like this:
#AllArgsConstructor(suppressConstructorProperties = true)
Now it works as i expected:
Json to send to http://localhost:8080/api/users:
{
"userName":"Anton",
"office":"http://localhost:8080/api/offices/1"
}
Which returns:
{
"userName":"Anton",
"_links": {
"self": {
"href": "http://localhost:8080/api/users/28"
},
"user": {
"href": "http://localhost:8080/api/users/28"
},
"office": {
"href": "http://localhost:8080/api/users/28/office"
}
}
}