.NET Core开发日志——OData

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。