c# - .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);
}

这对我们非常有用。将整个项目升级到所有 .NET Core 2.0 和 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'.

这个异常发生在这个 block 中:

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

如果我删除预先加载并像这样设置它，那么错误就会消失，但我也没有得到预期的结果:

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

我检查了 Microsoft Documentation为此，我没有看到那里显示任何表明我们在预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 Include 被忽略(阅读关于忽略的包含 here )。

如果“预期结果”是指您希望 record.PaperRecord.Paper 具有一个值，只需将其包含在您的投影中即可； EF 将建立关系。

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