Java 日報にコメントをつけたい

eclipseを使って、従業員日報管理システムを作っています。
従業員に日報に対して、コメントをつけられる機能を追加したいのですが、送信ボタンを押すとエラーが出てしまいました。

new.jspにreport_idの値が含まれていないからだと思っています。が、やり方がわかりません。
エラーを出なくするにはどのようにしたら良いでしょうか？

エラーメッセージ

HTTPステータス 500 - org.hibernate.PropertyValueException: not-null property references a null or transient value : models.Comment.report

servlet

package controller.comments;

import java.io.IOException;
import java.sql.Date;
import java.sql.Timestamp;
import java.util.List;

import javax.persistence.EntityManager;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import models.Employee;
import models.Report;
import models.Comment;
import models.validators.CommentValidator;
import utils.DBUtil;

/**
 * Servlet implementation class CreateCommentServlet
 */
@WebServlet("/comment/create")
public class CreateCommentServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public CreateCommentServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String _token = request.getParameter("_token");

        if (_token != null && _token.equals(request.getSession().getId())) {
            EntityManager em = DBUtil.createEntityManager();

            Comment c = new Comment();
            Report r = em.find(Report.class, (Integer.parseInt(request.getParameter("report.id"))));

            c.setReport(r);
            c.setEmployee((Employee) request.getSession().getAttribute("login_employee"));
            Date comment_date = new Date(System.currentTimeMillis());
            String cd_str = request.getParameter("comment_date");
            if (cd_str != null && !cd_str.equals("")) {
                comment_date = Date.valueOf(request.getParameter("comment_date"));
            }

            c.setComment_date(comment_date);
            c.setContent(request.getParameter("content"));
           c.setComment_date(Date.valueOf(request.getParameter("comment_date")));
            Timestamp currentTime = new Timestamp(System.currentTimeMillis());
            c.setCreated_at(currentTime);
            c.setUpdated_at(currentTime);

            List<String> errors = CommentValidator.validate(c);
            if (errors.size() > 0) {
                em.close();

                request.setAttribute("_token", request.getSession().getId());
                request.setAttribute("comment", c);
                request.setAttribute("errors", errors);
                request.setAttribute("report", r);
                RequestDispatcher rd = request.getRequestDispatcher("/WEB-INF/views/comment/new.jsp");
                rd.forward(request, response);
            } else {
                em.getTransaction().begin();
                em.persist(c);
                em.getTransaction().commit();
                em.close();
                request.getSession().setAttribute("flush", "登録が完了しました。");

                response.sendRedirect(request.getContextPath() + "/reports/index");
            }
        }
    }

}

jsp

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<c:import url="/WEB-INF/views/layout/app.jsp">
    <c:param name="content">
        <h2>コメント作成ページ</h2>

        <form method="POST" action="<c:url value='/comment/create' />">
            <c:import url="_form2.jsp" />
        </form>

        <p><a href="<c:url value='/reports/index' />">一覧に戻る</a></p>
    </c:param>
</c:import>

###_form2

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
<c:if test="${errors != null}">
    <div id="flush_error">
        入力内容にエラーがあります。<br />
        <c:forEach var="error" items="${errors}">
            ・<c:out value="${error}" /><br />
        </c:forEach>

    </div>
</c:if>
<label for="comment_date">日付</label><br />
<input type="date" name="comment_date" value="<fmt:formatDate value='${comment.comment_date}' pattern='yyyy-MM-dd' />" />
<br /><br />

<label for="name">氏名</label><br />
<c:out value="${sessionScope.login_employee.name}" />
<br /><br />



<label for="content">内容</label><br />
<textarea name="content" rows="10" cols="50">${comment.content}</textarea>
<br /><br />

<input type="hidden" name="_token" value="${_token}" />
<button type="submit">投稿</button>

model

package models;

import java.sql.Date;
import java.sql.Timestamp;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.Lob;
import javax.persistence.ManyToOne;
import javax.persistence.NamedQueries;
import javax.persistence.NamedQuery;
import javax.persistence.Table;

@Table(name="comments")
@NamedQueries({
    @NamedQuery(
        name = "getComments",
        query = "SELECT c FROM Comment AS c ORDER BY c.id DESC"
    ),
    @NamedQuery(
        name = "getCommentsCount",
        query = "SELECT COUNT(c) FROM Comment AS c"
    ),
})

@Entity
public class Comment {
@Id
@Column(name="id")
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;

@ManyToOne
@JoinColumn(name="employee_id", nullable= false )
private Employee employee;

@ManyToOne
@JoinColumn(name="report_id", nullable= false )
private Report report;

@Column(name = "comment_date", nullable = false)
private Date comment_date;

@Lob
@Column(name="content",nullable= false)
private String content;

@Column(name = "created_at", nullable = false)
private Timestamp created_at;

@Column(name = "updated_at", nullable = false)
private Timestamp updated_at;

public Integer getId() {
    return id;
}

public void setId(Integer id) {
    this.id = id;
}

public Employee getEmployee() {
    return employee;
}

public void setEmployee(Employee employee) {
    this.employee = employee;
}

public String getContent() {
    return content;
}

public void setContent(String content) {
    this.content = content;
}

public Timestamp getCreated_at() {
    return created_at;
}

public void setCreated_at(Timestamp created_at) {
    this.created_at = created_at;
}

public Timestamp getUpdated_at() {
    return updated_at;
}

public void setUpdated_at(Timestamp updated_at) {
    this.updated_at = updated_at;
}

public Date getComment_date() {
    return comment_date;
}

public void setComment_date(Date comment_date) {
    this.comment_date = comment_date;
}

public Report getReport() {
    return report;
}

public void setReport(Report report) {
    this.report = report;
}


}