简述
OData，即Open Data Protocol，是由微软在2007年推出的一款开放协议，旨在通过简单、标准的方式创建和使用查询式及交互式RESTful API。
类库
在.NET Core中想要使用OData功能的话需要添加Microsoft.AspNetCore.OData包。
dotnet add package Microsoft.AspNetCore.OData
准备模型类
public class Address
{
public string City { get; set; }
public string Street { get; set; }
}
public enum Category
{
Book,
Magazine,
EBook
}
public class Press
{
public int Id { get; set; }
public string Name { get; set; }
public string Email { get; set; }
public Category Category { get; set; }
}
public class Book
{
public int Id { get; set; }
public string ISBN { get; set; }
public string Title { get; set; }
public string Author { get; set; }
public decimal Price { get; set; }
public Address Address { get; set; }
public Press Press { get; set; }
}
创建Edm模型
OData使用EDM，即Entity Data Model来描述数据的结构。在Startup文件中添加创建方法。
private static IEdmModel GetEdmModel()
{
var builder = new ODataConventionModelBuilder();
builder.EntitySet<Book>("Books");
builder.EntitySet<Press>("Presses");
return builder.GetEdmModel();
}
注册OData服务
在Startup文件的ConfigureServices方法里注册OData服务。
services.AddOData();
services.AddMvc(options =>
{
options.EnableEndpointRouting = false;
}).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
这里要注意的是在.NET Core 2.2里，默认已经有终结点，所以要使用OData的终结点的话需要将默认选项禁用掉。
注册OData终结点
同样在Startup文件里，在其Configure方法内将原来的注册路由内容改为注册OData的终结点。
app.UseMvc(b =>
{
b.MapODataServiceRoute("odata", "odata", GetEdmModel());
});
显示元数据
运行程序后访问https://localhost:5001/odata/$metadata地址，可以看到所有可用模型的元数据。
<edmx:Edmx xmlns:edmx="http://docs.oasis-open.org/odata/ns/edmx" Version="4.0">
<edmx:DataServices>
<Schema xmlns="http://docs.oasis-open.org/odata/ns/edm" Namespace="Default">
<EntityType Name="Book">
<Key>
<PropertyRef Name="Id"/>
</Key>
<Property Name="Id" Type="Edm.Int32" Nullable="false"/>
<Property Name="ISBN" Type="Edm.String"/>
<Property Name="Title" Type="Edm.String"/>
<Property Name="Author" Type="Edm.String"/>
<Property Name="Price" Type="Edm.Decimal" Nullable="false"/>
<Property Name="Address" Type="Default.Address"/>
<NavigationProperty Name="Press" Type="Default.Press"/>
</EntityType>
<EntityType Name="Press">
<Key>
<PropertyRef Name="Id"/>
</Key>
<Property Name="Id" Type="Edm.Int32" Nullable="false"/>
<Property Name="Name" Type="Edm.String"/>
<Property Name="Email" Type="Edm.String"/>
<Property Name="Category" Type="Default.Category" Nullable="false"/>
</EntityType>
<ComplexType Name="Address">
<Property Name="City" Type="Edm.String"/>
<Property Name="Street" Type="Edm.String"/>
</ComplexType>
<EnumType Name="Category">
<Member Name="Book" Value="0"/>
<Member Name="Magazine" Value="1"/>
<Member Name="EBook" Value="2"/>
</EnumType>
<EntityContainer Name="Container">
<EntitySet Name="Books" EntityType="Default.Book">
<NavigationPropertyBinding Path="Press" Target="Presses"/>
</EntitySet>
<EntitySet Name="Presses" EntityType="Default.Press"/>
</EntityContainer>
</Schema>
</edmx:DataServices>
</edmx:Edmx>
创建Controller
本文实例中不考虑数据库的操作，故而使用hard code方式构建必要的模型对象。
public class BooksController : ODataController
{
private static IList<Book> Books {get; set;}
public BooksController()
{
Books = new List<Book>
{
new Book
{
Id = 1,
ISBN = "111-0-321-56789-1",
Title = "Calculus",
Price = 66.6m,
Address = new Address
{
City = "Shanghai",
Street = "Beijin Xi Road"
},
Press = new Press
{
Id = 1,
Name = "Shanghai Tongji",
Category = Category.Book
}
},
new Book
{
Id = 2,
ISBN = "222-2-654-00000-2",
Title = "Linear Algebra",
Price = 53.2m,
Address = new Address
{
City = "Shanghai",
Street = "Beijin Dong Road"
},
Press = new Press
{
Id = 2,
Name = "Shanghai Fudan",
Category = Category.EBook
}
}
};
}
[EnableQuery]
public IActionResult Get()
{
return Ok(Books);
}
[EnableQuery]
public IActionResult Get(int key)
{
return Ok(Books.FirstOrDefault(b => b.Id == key));
}
}
EnableQuery特性在需要高级查询的场景时必须添加。
查询
加入Controller之后，访问https://localhost:5001/odata/Books地址，可得到所有Book数据。
{
"@odata.context": "https://localhost:5001/odata/$metadata#Books",
"value": [
{
"Id": 1,
"ISBN": "111-0-321-56789-1",
"Title": "Calculus",
"Author": null,
"Price": 66.6,
"Address": {
"City": "Shanghai",
"Street": "Beijin Xi Road"
}
},
{
"Id": 2,
"ISBN": "222-2-654-00000-2",
"Title": "Linear Algebra",
"Author": null,
"Price": 53.2,
"Address": {
"City": "Shanghai",
"Street": "Beijin Dong Road"
}
}
]
}
访问https://localhost:5001/odata/Books(1)地址，可得到key值为1的Book数据。
{
"@odata.context": "https://localhost:5001/odata/$metadata#Books/$entity",
"Id": 1,
"ISBN": "111-0-321-56789-1",
"Title": "Calculus",
"Author": null,
"Price": 66.6,
"Address": {
"City": "Shanghai",
"Street": "Beijin Xi Road"
}
}
高级查询
如果想要使用OData查询的高级功能，可以在注册终结点时额外加上相应的配置。
app.UseMvc(b =>
{
b.Select().Expand().Filter().OrderBy().MaxTop(100).Count();
b.MapODataServiceRoute("odata", "odata", GetEdmModel());
});
访问网址时加上所需的查询内容：
https://localhost:5001/odata/Books?$select=Id,Title
{
"@odata.context": "https://localhost:5001/odata/$metadata#Books(Id,Title)",
"value": [
{
"Id": 1,
"Title": "Calculus"
},
{
"Id": 2,
"Title": "Linear Algebra"
}
]
}
如果想要按特定条件过滤数据内容的话也很容易：
https://localhost:5001/odata/Books?$filter=Price%20le%2060
{
"@odata.context": "https://localhost:5001/odata/$metadata#Books",
"value": [
{
"Id": 2,
"ISBN": "222-2-654-00000-2",
"Title": "Linear Algebra",
"Author": null,
"Price": 53.2,
"Address": {
"City": "Shanghai",
"Street": "Beijin Dong Road"
}
}
]
}
总结
不难看出，OData的真正魅力在于其对那些高级查询功能的支持，所以在创建RESTful API时，不妨考虑使用OData，这样应该能减少许多不必要的代码工作。
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