.NET Core/EF Core 2.0 升级后急切加载 "No coercion operator is defined between types"

Question

在 .NET Core 1.x 中，我们有一个如下所示的方法：

public async Task<(PaperRecord Component, PaperPointerRecord Pointer)> GetOneAsync(DbSet<PaperPointerRecord> paperPointerRecords,
                                                                                   int? paperId = null,
                                                                                   long? externalPaperId = null,
                                                                                   Expression<Func<PaperPointerRecord, bool>> filter = null)
{
    var query = filter is null ? paperPointerRecords.AsQueryable() : paperPointerRecords.Where(filter);

    if (paperId.HasValue)
        query = query.Where(_ => _.PaperPointerId == paperId);

    if (externalPaperId.HasValue)
        query = query.Where(_ => _.ExternalId == externalPaperId);

    var record = await query.Include(_ => _.Paper)
                            .ThenInclude(_ => _.PaperColors)
                            .Select(_ => new
                                         {
                                             PaperRecord = _.Paper,
                                             PaperPointerRecord = _
                                         })
                            .SingleOrDefaultAsync();

    return !(record is null) ? (record.PaperRecord, record.PaperPointerRecord) : throw NewPaperRecordNotFoundException(paperId, externalPaperId);
}

这对我们很有用。将整个项目升级到所有.NET Core 2.0 & EF Core 2.0后，该方法抛出此异常：

System.InvalidOperationException occurred: No coercion operator is defined between types 'PaperPointerRecord' and 'PaperRecord'.

此块中出现此异常：

var record = await query.Include(_ => _.Paper)
                        .ThenInclude(_ => _.PaperColors)
                        .Select(_ => new
                                     {
                                         PaperRecord = _.Paper,
                                         PaperPointerRecord = _
                                     })
                        .SingleOrDefaultAsync();

如果我删除预加载并像这样设置，错误就会消失，但我也没有得到预期的结果：

var record = await query.Select(_ => new
                                     {
                                         PaperRecord = _.Paper,
                                         PaperPointerRecord = _
                                     })
                        .SingleOrDefaultAsync();

我已经为此检查了 Microsoft Documentation，但我没有看到那里显示任何表明我们在预先加载方面做错了什么。

Answer 1

Include 在这种情况下没有用，因为您正在返回一个投影并且 Includes 被忽略（阅读关于被忽略的包括 here）。

如果 "intended results" 你的意思是你希望 record.PaperRecord.Paper 有一个值，只需将它包含在你的投影中； EF 将建立关系。

new
{
    PaperRecord = _.Paper,
    PaperPointerRecord = _,
    PaperColor = _.Paper.PaperColor,
}

.NET Core/EF Core 2.0 升级后急切加载 "No coercion operator is defined between types"

Eager Loading "No coercion operator is defined between types" after .NET Core/EF Core 2.0 Upgrade

c#

linq

eager-loading

entity-framework-core

.net-core