Spring Data REST call to `save` receiving object with null variable

I'm continuing the development of a system that uses the React JavaScript library (and related stuff) on the front end and Spring Data REST, Hibernate, PostgreSQL and related stuff on the back end.

This system will be used by people who may own one or more companies and their clients. This means that most/all model objects will have a reference to the Company(ies) that they belong to. Also, company owners will have a few Employees that will have higher level access on this system (or these will be the owners themselves).

I need to implement a functionality where, when a company is inserted in the database, an employee is inserted as well. Also, if one fails, both must fail. Because of how the model was set up, I'm sending an Employee object to be saved, and, within it, the new Company, like this (using Axios):

employee: {
    // ...,
    company: {
        // ....
    }
}

Problem is, when the save method is called in the back end, the Company member of the Employee object is null. I've tried a few things, like messing with the relationship, adding an Employee list to the Company object, passing the Company object separately, but nothing worked.

What else could I try? Here are some classes:

Record.java

package xxx.model.common;

import javax.persistence.Column;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;
import javax.validation.constraints.NotNull;

import lombok.Data;

@Data
@MappedSuperclass
public abstract class Record {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    protected Long id;

    @NotNull
    @Column(name = "deleted")
    protected Boolean isDeleted = false;

    @NotNull
    @Column(name = "enabled")
    protected Boolean isEnabled = true;
}

Company.java

package xxx.model;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.AttributeOverride;
import javax.persistence.AttributeOverrides;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.Table;
import javax.validation.constraints.NotBlank;

import com.fasterxml.jackson.annotation.JsonManagedReference;
import xxx.common.Record;
// ...

import lombok.Data;
import lombok.EqualsAndHashCode;

@Data
@EqualsAndHashCode(callSuper=false)
@Entity
@Table(name="company")
@AttributeOverrides( { @AttributeOverride(name = "id", column = @Column(name = "id_company")) } )
public class Company extends Record {

    /*
     * ...
     */

    // Necessary for Hibernate
    protected Company() {}

    public Company(/* ... */) {
        /*
         * ...
         */
    }
}

Registry.java

package xxx.model.common;

import javax.persistence.Column;
import javax.persistence.MappedSuperclass;
import javax.validation.constraints.NotBlank;

import lombok.Data;
import lombok.EqualsAndHashCode;

@Data
@EqualsAndHashCode(callSuper=false)
@MappedSuperclass
public abstract class Registry extends Record {

    @NotBlank
    @Column(name = "code", length = 15)
    protected String code;

    @NotBlank
    @Column(name = "name", length = 40)
    protected String name;
}

RegistrySingleCompany.java

package xxx.model.common;

import javax.persistence.CascadeType;
import javax.persistence.FetchType;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.MappedSuperclass;

import com.fasterxml.jackson.annotation.JsonBackReference;
import xxx.model.Company;

import lombok.Data;
import lombok.EqualsAndHashCode;

@Data
@EqualsAndHashCode(callSuper=false)
@MappedSuperclass
public class RegistrySingleCompany extends Registry {

    @ManyToOne(fetch = FetchType.LAZY, cascade = { CascadeType.MERGE }, optional= false)
    @JoinColumn(name="id_company")
    protected Company company;
}

Employee.java

package xxx.model;

import javax.persistence.AttributeOverride;
import javax.persistence.AttributeOverrides;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToOne;
import javax.persistence.Table;

import xxx.model.common.RegistrySingleCompany;

import lombok.Data;
import lombok.EqualsAndHashCode;

@Data
@EqualsAndHashCode(callSuper=false)
@Entity
@Table(name="employee")
@AttributeOverrides( { @AttributeOverride(name = "id", column = @Column(name = "id_employee")) } )
public class Employee extends RegistrySingleCompany {

    /*
     * ...
     */

    // Necessary for Hibernate
    protected Employee() {}
}

EmployeeRepositoryCustom.java

package xxx.repository.custom;

import org.springframework.data.repository.query.Param;

import xxx.model.Employee;

public interface EmployeeRepositoryCustom {

    <S extends Employee> S save(S entity);
}

EmployeeRepositoryCustomImpl.java

package xxx.repository.custom;

import javax.persistence.PersistenceContext;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.util.Assert;
import org.springframework.web.bind.annotation.RequestBody;

import xxx.model.Employee;

public class EmployeeRepositoryCustomImpl implements EmployeeRepositoryCustom {

    @Override
    @Transactional
    public <S extends Employee> S save(@RequestBody S entity) {
        /*
         * ...
         */
        return entity;
    }
}

EmployeeProjection.java

package xxx.model.projection;

import org.springframework.data.rest.core.config.Projection;

import xxx.model.Employee;

@Projection(name = "employeeProjection", types = { Employee.class }) 
public interface EmployeeProjection {

    Boolean getIsDeleted();

    Boolean getIsEnabled();

    String getCode();

    String getName();

    /*
     * ...
     */
}

EmployeeRepository.java

package xxx.repository;

import org.springframework.data.repository.PagingAndSortingRepository;
import org.springframework.data.rest.core.annotation.RepositoryRestResource;

import xxx.model.Employee;
import xxx.model.projection.EmployeeProjection;
import xxx.repository.custom.EmployeeRepositoryCustom;

@RepositoryRestResource(collectionResourceRel = "employee", path = "employees", excerptProjection = EmployeeProjection.class)
public interface EmployeeRepository extends PagingAndSortingRepository<Employee, Long>, EmployeeRepositoryCustom {}

Thanks in advance.

Edit: added missing classes.