Child child = new Child();
   Parent parent = new Parent();
   parent.setParentId(idOfExistingParent); //Parent with this id already exists in DB
   child.setParent(parent);
   child.setModifyBy(usrSeqId);
   child.setModifyTimstm(currentTime);
    try {
      entityManager.persist(bidlistOffer);
    } catch (PersistenceException persistenceException) {
      log.error(persistenceException.getMessage());
    }

import java.math.BigDecimal;
import java.sql.Timestamp;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.SequenceGenerator;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity
@Table(name = "PARENT", uniqueConstraints = @UniqueConstraint(columnNames = {
    "ASSOC_PARENT_1_ID", "ASSOC_PARENT_2_ID", "ASSOC_PARENT_3_ID" }))
public class FinanceSourceCreditLimit implements java.io.Serializable {

  private long parentId;
  private AssocParent1 assocParent1;
  private AssocParent2 assocParent2;
  private AssocParent3 assocParent3;
  private BigDecimal decColumn1;
  private Set<Child> childSet = new HashSet<>();
  private long modifyById;
  private Timestamp modifyTimstm;

  public Parent() {
    /**
     * Any entity class must define a non-private zero-argument constructor
     */
  }

  public Parent(long parentId,
      AssocParent1 assocParent1, AssocParent2 assocParent2,
      AssocParent3 assocParent3, BigDecimal decColumn1, Set<Child> children,
      long modifyById, Timestamp modifyTimstm) {
    this.parentId = parentId;
    this.assocParent1 = assocParent1;
    this.assocParent2 = assocParent2;
    this.assocParent3 = assocParent3;
    this.decColumn1 = decColumn1;
    this.children = children;
    this.modifyById = modifyById;
    this.modifyTimstm = modifyTimstm;
  }

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "ParentIdGenerator")
  @SequenceGenerator(name = "ParentIdGenerator", sequenceName = "SEQ_PARENT", allocationSize = 1)
  @Column(name = "PARENT_ID", unique = true, nullable = false, precision = 10, scale = 0)
  public long getParentId() {
    return this.parentId;
  }

  public void setParentId(long parentId) {
    this.parentId = parentId;
  }

  @ManyToOne(fetch = FetchType.LAZY)
  @JoinColumn(name = "ASSOC_PARENT_1_ID", nullable = false)
  public AssocParent1 getAssocParent1() {
    return this.assocParent1;
  }

  public void setCreditLimitType(AssocParent1 assocParent1) {
    this.assocParent1 = assocParent1;
  }

  @ManyToOne(fetch = FetchType.LAZY)
  @JoinColumn(name = "ASSOC_PARENT_2_ID")
  public AssocParent2 getAssocParent2() {
    return this.assocParent2;
  }

  public void setAssocParent2(AssocParent2 assocParent2) {
    this.assocParent2 = assocParent2;
  }

  @ManyToOne(fetch = FetchType.LAZY)
  @JoinColumn(name = "ASSOC_PARENT_3_ID", nullable = false)
  public AssocParent3 getAssocParent3() {
    return this.assocParent3;
  }

  public void setAssocParent3(AssocParent3 assocParent3) {
    this.assocParent3 = assocParent3;
  }

  @Column(name = "DEC_COL_1", precision = 10)
  public BigDecimal getDecColumn1() {
    return this.decColumn1;
  }

  public void setDecColumn1(BigDecimal decColumn1) {
    this.decColumn1 = decColumn1;
  }

  @OneToMany(fetch = FetchType.LAZY, mappedBy = "parent")
  @OptimisticLock(excluded=true)
  public Set<Child> getChildren() {
    return this.children;
  }

  public void setChildren(Set<Child> children) {
    this.children = children;
  }

  @Column(name = "MODIFY_BY_ID", nullable = false, precision = 10, scale = 0)
  public long getModifyById() {
    return this.modifyById;
  }

  public void setModifyById(long modifyById) {
    this.modifyById = modifyById;
  }

  @Version
  @org.hibernate.annotations.Generated(GenerationTime.ALWAYS)
  @org.hibernate.annotations.Source(SourceType.VM)
  @Column(name = "MODIFY_TIMSTM", nullable = false)
  public Timestamp getModifyTimstm() {
    return this.modifyTimstm;
  }

  public void setModifyTimstm(Timestamp modifyTimstm) {
    this.modifyTimstm = modifyTimstm;
  }

}

import java.sql.Timestamp;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.SequenceGenerator;
import javax.persistence.Table;
import javax.persistence.Version;

@Entity
@Table(name = "Child")
public class Child implements java.io.Serializable {

  private long                     childId;
  private Parent                parent;
  private long                     modifyBy;
  private Timestamp                modifyTimstm;

  public Child() {
    /**
     * Any entity class must define a non-private zero-argument constructor
     */
  }

  public Child(long childId, Parent parent, long modifyBy, Timestamp modifyTimstm) {
    this.childId = childId;
    this.parent = parent;
    this.modifyBy = modifyBy;
    this.modifyTimstm = modifyTimstm;
  }

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SEQ_CHILD")
  @SequenceGenerator(name = "SEQ_CHILD", sequenceName = "SEQ_CHILD", allocationSize = 1)
  @Column(name = "CHILD_ID", unique = true, nullable = false, precision = 10, scale = 0)
  public long getChildId() {
    return this.childId;
  }

  public void setChildId(long childId) {
    this.childId = childId;
  }

  @ManyToOne(fetch = FetchType.LAZY)
  @JoinColumn(name = "PARENT_ID", nullable = true) //modified
  //@JoinColumn(name = "PARENT_ID", nullable = false)
  @OptimisticLock(excluded=true)
  public Parent getParent() {
    return this.parent;
  }

  public void setParent(Parent parent) {
    this.parent = parent;
  }

  @Column(name = "MODIFY_BY", nullable = false, precision = 10, scale = 0)
  public long getModifyBy() {
    return this.modifyBy;
  }

  public void setModifyBy(long modifyBy) {
    this.modifyBy = modifyBy;
  }

  @Version
  @org.hibernate.annotations.Generated(GenerationTime.ALWAYS)
  @org.hibernate.annotations.Source(SourceType.VM)
  @Column(name = "MODIFY_TIMSTM", nullable = false)
  public Timestamp getModifyTimstm() {
    return this.modifyTimstm;
  }

  public void setModifyTimstm(Timestamp modifyTimstm) {
    this.modifyTimstm = modifyTimstm;
  }

}