Canvas中的category获取不到值

CanvasDao的selectAll方法，Canvas的普通属性都能获取到值，但category是空，什么原因

Category类

public class Category {
   private Integer id;
   private String name;
   private String createName;
   private Date createTime;
   private Date updateTime;
   private String description;

   public Category() {
   }

   public Category(Integer id, String name, String createName, Date createTime, Date updateTime, String description) {
       this.id = id;
       this.name = name;
       this.createName = createName;
       this.createTime = createTime;
       this.updateTime = updateTime;
       this.description = description;
   }

   public Integer getId() {
       return id;
   }

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

   public String getName() {
       return name;
   }

   public void setName(String name) {
       this.name = name;
   }

   public String getCreateName() {
       return createName;
   }

   public void setCreateName(String createName) {
       this.createName = createName;
   }

   public Date getCreateTime() {
       return createTime;
   }

   public void setCreateTime(Date createTime) {
       this.createTime = createTime;
   }

   public Date getUpdateTime() {
       return updateTime;
   }

   public void setUpdateTime(Date updateTime) {
       this.updateTime = updateTime;
   }

   public String getDescription() {
       return description;
   }

   public void setDescription(String description) {
       this.description = description;
   }
}

Canvas类

public class Canvas {
private Integer id;
private Integer categoryId;
private String name;
private String creator;
private String imgPath;
private Date createTime;
private Date updateTime;
private String description;
private String details;
private Category category;

   public Canvas() {
   }

   public Canvas(Integer id, Integer categoryId, String name, String creator, String imgPath, Date createTime, Date updateTime, String description, String details, Category category) {
       this.id = id;
       this.categoryId = categoryId;
       this.name = name;
       this.creator = creator;
       this.imgPath = imgPath;
       this.createTime = createTime;
       this.updateTime = updateTime;
       this.description = description;
       this.details = details;
       this.category = category;
   }

   public Integer getId() {
       return id;
   }

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

   public Integer getCategoryId() {
       return categoryId;
   }

   public void setCategoryId(Integer categoryId) {
       this.categoryId = categoryId;
   }

   public String getName() {
       return name;
   }

   public void setName(String name) {
       this.name = name;
   }

   public String getCreator() {
       return creator;
   }

   public void setCreator(String creator) {
       this.creator = creator;
   }

   public String getImgPath() {
       return imgPath;
   }

   public void setImgPath(String imgPath) {
       this.imgPath = imgPath;
   }

   public Date getCreateTime() {
       return createTime;
   }

   public void setCreateTime(Date createTime) {
       this.createTime = createTime;
   }

   public Date getUpdateTime() {
       return updateTime;
   }

   public void setUpdateTime(Date updateTime) {
       this.updateTime = updateTime;
   }

   public String getDescription() {
       return description;
   }

   public void setDescription(String description) {
       this.description = description;
   }

   public String getDetails() {
       return details;
   }

   public void setDetails(String details) {
       this.details = details;
   }

   public Category getCategory() {
       return category;
   }

   public void setCategory(Category category) {
       this.category = category;
   }

   @Override
   public String toString() {
       return "Canvas{" +
               "id=" + id +
               ", categoryId=" + categoryId +
               ", name='" + name + '\'' +
               ", creator='" + creator + '\'' +
               ", imgPath='" + imgPath + '\'' +
               ", createTime=" + createTime +
               ", updateTime=" + updateTime +
               ", description='" + description + '\'' +
               ", details='" + details + '\'' +
               ", category=" + category +
               '}';
   }
}

CanvasDao类

public interface CanvasDao {

   @Select("select a.*,c.id cId,c.name cName,c.createTime cCreateTime,c.updateTime cUpdateTime,c.description cDescription from canvas a left join category c on a.categoryId=c.id;")
   @Results(value = {
           @Result(column = "id",property = "id",id = true),
           @Result(column = "categoryId",property = "categoryId"),
           @Result(column = "name",property = "name"),
           @Result(column = "creator",property = "creator"),
           @Result(column = "price",property = "price"),
           @Result(column = "imgPath",property = "imgPath"),
           @Result(column = "createTime",property = "createTime"),
           @Result(column = "updateTime",property = "updateTime"),
           @Result(column = "description",property = "description"),
           @Result(column = "details",property = "details"),
           @Result(column = "cId",property = "category.id"),
           @Result(column = "cName",property = "category.name"),
           @Result(column = "cCreateName",property = "category.createName"),
           @Result(column = "cCreateTime",property = "category.createTime"),
           @Result(column = "cUpdateTime",property = "category.updateTime"),
           @Result(column = "cDescription",property = "category.description")
   })
   List<Canvas> selectAll();

}