EF&自动映射器.更新嵌套集合 [英] EF & Automapper. Update nested collections

问题描述

我试图更新Country实体的嵌套集合(城市).

I trying to update nested collection (Cities) of Country entity.

只是简单的实体和dto:

Just simple enitities and dto's:

// EF Models
public class Country
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<City> Cities { get; set; }
}

public class City
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CountryId { get; set; }
    public int? Population { get; set; }

    public virtual Country Country { get; set; }
}

// DTo's
public class CountryData : IDTO
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<CityData> Cities { get; set; }
}

public class CityData : IDTO
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CountryId { get; set; }
    public int? Population { get; set; }
}

和代码本身(为简单起见在控制台应用程序中经过测试):

And code itself (tested in console app for the sake of simplicity):

        using (var context = new Context())
        {
            // getting entity from db, reflect it to dto
            var countryDTO = context.Countries.FirstOrDefault(x => x.Id == 1).ToDTO<CountryData>();

            // add new city to dto 
            countryDTO.Cities.Add(new CityData 
                                      { 
                                          CountryId = countryDTO.Id, 
                                          Name = "new city", 
                                          Population = 100000 
                                      });

            // change existing city name
            countryDTO.Cities.FirstOrDefault(x => x.Id == 4).Name = "another name";

            // retrieving original entity from db
            var country = context.Countries.FirstOrDefault(x => x.Id == 1);

            // mapping 
            AutoMapper.Mapper.Map(countryDTO, country);

            // save and expecting ef to recognize changes
            context.SaveChanges();
        }

此代码引发异常:

操作失败:由于一个或多个外键属性不可为空，因此无法更改该关系.对关系进行更改时，相关的外键属性将设置为空值.如果外键不支持空值，则必须定义新的关系，必须为外键属性分配另一个非空值，或者必须删除不相关的对象.

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

即使最后一次映射后的实体看起来很好，并且可以正确反映所有更改.

even though entity after last mapping seems just fine and reflects all changes properly.

我花了很多时间寻找解决方案，但没有结果.请帮忙.

I've spent much time finding solution but got no result. Please help.

推荐答案

问题是从数据库中检索的country已经有一些城市.当您像这样使用AutoMapper时:

The problem is the country you are retrieving from database already has some cities. When you use AutoMapper like this:

// mapping 
        AutoMapper.Mapper.Map(countryDTO, country);

AutoMapper正在执行类似正确创建IColletion<City>的操作(在示例中为一个城市)，然后将此全新的收藏集分配给您的country.Cities属性.

AutoMapper is doing something like creating an IColletion<City> correctly (with one city in your example), and assigning this brand new collection to your country.Cities property.

问题在于EntityFramework不知道如何处理旧的城市.

The problem is EntityFramework doesn't know what to do with the old collection of cities.

• 是否应该删除您的旧城市并仅采用新的收藏?
• 是否应该将两个列表合并并保存在数据库中?

实际上，EF无法为您做出决定.如果您想继续使用AutoMapper，可以自定义映射，如下所示:

In fact, EF cannot decide for you. If you want to keep using AutoMapper, you can customize your mapping like this:

// AutoMapper Profile
public class MyProfile : Profile
{

    protected override void Configure()
    {

        Mapper.CreateMap<CountryData, Country>()
            .ForMember(d => d.Cities, opt => opt.Ignore())
            .AfterMap(AddOrUpdateCities);
    }

    private void AddOrUpdateCities(CountryData dto, Country country)
    {
        foreach (var cityDTO in dto.Cities)
        {
            if (cityDTO.Id == 0)
            {
                country.Cities.Add(Mapper.Map<City>(cityDTO));
            }
            else
            {
                Mapper.Map(cityDTO, country.Cities.SingleOrDefault(c => c.Id == cityDTO.Id));
            }
        }
    }
}

用于Cities的Ignore()配置使AutoMapper只保留由EntityFramework构建的原始代理引用.

The Ignore() configuration used for Cities makes AutoMapper just keep the original proxy reference built by EntityFramework.

然后，我们只使用AfterMap()来执行一个您完全想做的动作:

Then we just use AfterMap() to invoke an action doing exactly what you thougth:

• 对于新城市，我们从 DTO 映射到实体(AutoMapper创建了一个新 实例)并将其添加到国家/地区的收藏集中.
• 对于现有城市，我们使用Map的重载，其中将现有实体作为第二个参数，并将城市代理作为第一个参数，因此automapper只会更新现有实体的属性.

• For new cities, we map from DTO to Entity (AutoMapper creates a new instance) and add it to country's collection.
• For existing cities, we use an overload of Map where we pass the existing entity as the second parameter, and the city proxy as first parameter, so automapper just updates the existing entity's properties.

然后您可以保留原始代码:

Then you can keep your original code:

using (var context = new Context())
    {
        // getting entity from db, reflect it to dto
        var countryDTO = context.Countries.FirstOrDefault(x => x.Id == 1).ToDTO<CountryData>();

        // add new city to dto 
        countryDTO.Cities.Add(new CityData 
                                  { 
                                      CountryId = countryDTO.Id, 
                                      Name = "new city", 
                                      Population = 100000 
                                  });

        // change existing city name
        countryDTO.Cities.FirstOrDefault(x => x.Id == 4).Name = "another name";

        // retrieving original entity from db
        var country = context.Countries.FirstOrDefault(x => x.Id == 1);

        // mapping 
        AutoMapper.Mapper.Map(countryDTO, country);

        // save and expecting ef to recognize changes
        context.SaveChanges();
    }

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