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[英]NullPointerException on multi-table primary key JPA when building application
[英]Spring JPA multi-table relationships
我正在尝试创建具有产品->属性->选项的产品目录
因此数据看起来像这样:
product_one
attribute_one
option_one
option_two
attribute_two
option_one
option_two
该代码可在GitHub https://github.com/ccsalway/prod_info_mngr上找到
我为每个实体创建了一个类:
@Entity
class Product {
@Id
@Generatedvalue
private Long id;
private String name;
// getters and setters
}
@Entity
class Attribute {
@Id
@Generatedvalue
private Long id;
private String name;
@ManyToOne(cascade = CascadeType.REMOVE)
private Product product;
// getters and setters
}
@Entity
class Option {
@Id
@Generatedvalue
private Long id;
private String name;
@ManyToOne(cascade = CascadeType.REMOVE)
private Attribute attribute;
// getters and setters
}
我为每个实体创建了一个存储库:
@Repository
public interface ProductRepository extends PagingAndSortingRepository<Product, Long> {
}
@Repository
public interface AttributeRepository extends PagingAndSortingRepository<Attribute, Long> {
}
@Repository
public interface OptionRepository extends PagingAndSortingRepository<Option, Long> {
}
我为每个实体创建了一个服务:
@Service
public class ProductService {
// Autowired Repositories
// methods
}
@Service
public class AttributeService {
// Autowired Repositories
// methods
}
@Service
public class OptionService {
// Autowired Repositories
// methods
}
我为每个实体创建了一个控制器:
@Controller
@RequestMapping("/product")
public class ProductController {
@Autowired
private ProductService productService;
//methods
}
@Controller
@RequestMapping("/product/{prod_id}/attribute")
public class AttributeController{
@Autowired
private AttributeService attributeService;
//methods
}
@Controller
@RequestMapping("/product/{prod_id}/attribute/{attr_id}")
public class OptionController {
@Autowired
private OptionService optionService;
//methods
}
并且(最后)我为每个Controller创建了几个视图(我不会在这里添加它们)。
我想在product_view.jsp
视图中执行的操作是显示属性及其相关选项的列表,如下所示:
<table id="attrTable">
<thead>
<tr>
<th>Name</th>
<th>Options</th>
</tr>
</thead>
<tbody>
<c:forEach items="${attributes}" var="attr">
<tr data-id="${attr.id}">
<td>${fn:htmlEscape(attr.name)}</td>
<td><c:forEach items="${attr.options}" var="opt" varStatus="loop">
${opt.name}<c:if test="${!loop.last}">,</c:if>
</c:forEach></td>
</tr>
</c:forEach>
</tbody>
</table>
所以桌子看起来像这样
product_one
attribute_one option_one, option_two
attribute_two option_one, option_two
第一步是在ProductController
创建一个@RequestMapping
:
@RequestMapping(value = "/{id}", method = RequestMethod.GET)
public String view(Model model, @PathVariable Long id) {
Product product = productService.getProduct(id);
List<Attribute> attributes = productService.getAttributes(product);
model.addAttribute("product", product);
model.addAttribute("attributes", attributes);
return "products/product_view";
}
但是视图中的${attr.options}
无法识别options
键,那么如何进行这项工作?
我尝试在Product
实体中添加@OneToMany
关联,但这在数据库中使用product_id|attribute_id
创建了一个表,然后您必须保存该属性,然后使用新属性更新产品,这也意味着选择一个产品,您将提取所有属性和所有选项,以防止在这些属性上进行分页。
@Entity
class Product {
@Id
@Generatedvalue
private Long id;
private String name;
@OneToMany
List<Attribute> attributes;
// getters and setters
}
我找到了解决方案:
我添加了OneToMany
和ManyToOne
关系,如下所示。 此方法还允许repository.delete(id)
方法级联删除。
FetchType.LAZY告诉Spring仅在请求时获取基础项。 例如,当请求
Product
,将获取id
和name
,但是由于attributes
为@OneToMany
(默认情况下为LAZY),因此只有在特定调用时才从数据库中获取attributes
product.getAttributes()
已制成。
@Entity
public class Product {
@Id
@GeneratedValue
private Long id;
private String name;
// OneToMany is, by default, a LAZY fetch
@OneToMany(cascade = CascadeType.ALL, mappedBy = "product")
private Set<Attribute> attributes = new HashSet<>();
// getters and setters
}
@Entity
public class Attribute {
@Id
@GeneratedValue
private Long id;
private String name;
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "product_id", nullable = false)
private Product product;
// OneToMany is, by default, a LAZY fetch
@OneToMany(cascade = CascadeType.ALL, mappedBy = "attribute")
private Set<Option> options = new HashSet<>();
// getters and setters
}
@Entity
public class Option {
@Id
@GeneratedValue
private Long id;
@NotEmpty
@Size(min = 1, max = 32)
private String name;
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "attribute_id", nullable = false)
private Attribute attribute;
// getters and setters
}
在ProductController
,我将Attributes
与Product
分开,以便可以对属性使用Paging(而如果我只是调用product.getAttributes()
,它将获取所有属性)
@RequestMapping(value = "/{id}", method = RequestMethod.GET)
public String view(Model model, @PathVariable Long id, @RequestParam(name = "page", defaultValue = "0") int page) throws ProductNotFoundException {
Product product = productService.getProduct(id);
model.addAttribute("product", product);
// requesting attributes separately (as opposed to using LAZY) allows you to use paging
Page<Attribute> attributes = productService.getAttributes(product, new PageRequest(page, 10));
model.addAttribute("attributes", attributes);
return "products/product_view";
}
然后在视图中,我记得要遍历attributes
而不是如上所述的product.attributes
。
因为在Attribute
实体中options
属性设置为LAZY
,所以当循环调用attr.options
,Spring将向数据库请求当前Attribute
的Options
。
<table id="attrTable" class="table is-hoverable is-striped is-fullwidth" style="cursor:pointer;">
<thead>
<tr>
<th>Name</th>
<th>Options</th>
</tr>
</thead>
<tbody>
<c:forEach items="${attributes}" var="attr">
<tr data-id="${attr.id}">
<td>${fn:htmlEscape(attr.name)}</td>
<td>
<c:forEach items="${attr.options}" var="opt" varStatus="loop">
${opt.name}<c:if test="${!loop.last}">,</c:if>
</c:forEach>
</td>
</tr>
</c:forEach>
</tbody>
</table>
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