实体框架：在一个查询中从不同的表中加载数据

Question

在我使用 EF Core 5.0 的 C# 项目中，我有几个独立的表：Clothes、Hairs、Makeup。他们的一些列是相似的，但有些不是..` 我需要编写一个从这些表中加载行的方法。我现在拥有的是：

public async Task<(ClothesDbModel[] clothes, MakeupDbModel[] makeups, HairDbModel[] hairs)> GetDressup(int[] clothesIds, int[] makeupIds, int[] hairIds)
{
    ClothesDbModel[] clothes = new ClothesDbModel[0];
    if (clothesIds.Length > 0)
    {
        clothes = await _dbContext.Clothes.Where(c => clothesIds.Contains(c.Id)).ToArrayAsync();
    }

    MakeupDbModel[] makeups = new MakeupDbModel[0];
    if (makeupIds.Length > 0)
    {
        makeups = await _dbContext.Makeups.Where(c => makeupIds.Contains(c.Id)).ToArrayAsync();
    }

    HairDbModel[] hairs = new HairDbModel[0];
    if (hairIds.Length > 0)
    {
        hairs = await _dbContext.Hairs.Where(c => hairIds.Contains(c.Id)).ToArrayAsync();
    }

    return (clothes, makeups, hairs);
}

但是，在这种情况下，我对数据库有 3 个单独的查询（3 个等待）。从性能的角度来看，我认为这不是加载数据的最佳方式。也许我只能使用 DbContext 加载相同的数据一次&

Answer 1

我希望它可以与 EF 一起使用。但是我知道 Concat 有问题，所以它可能会失败。

class CombinedResult
{
    public ClothesDbModel Clothes;
    public MakeupDbModel Makeup;
    public HairDbModel Hair;
}

...

public async Task<(ClothesDbModel[] clothes, MakeupDbModel[] makeups, HairDbModel[] hairs)> GetDressup(int[] clothesIds, int[] makeupIds, int[] hairIds)
{
    var queries = new List<IQueryable<CombinedResult>>();

    if (clothesIds.Length > 0)
    {
        queries.Add(_dbContext.Clothes.Where(c => clothesIds.Contains(c.Id)).Select(c => new CombinedResult { Clothes = c }));
    }

    if (makeupIds.Length > 0)
    {
        queries.Add(_dbContext.Makeups.Where(c => makeupIds.Contains(c.Id)).Select(c => new CombinedResult { Makeup = c }));
    }

    if (hairIds.Length > 0)
    {
        queries.Add(_dbContext.Hairs.Where(c => hairIds.Contains(c.Id)).Select(c => new CombinedResult { Hair = c }));
    }

    var clothes = new List<ClothesDbModel>();
    var makeups = new List<MakeupDbModel>();
    var hairs = new List<HairDbModel>();

    if (queries.Count > 0)
    {
        var query = queries[0];
        for (var i = 1; i < queries.Count; i++)
        {
            query = query.Concat(queries[i]);
        }

        var items = await query.ToListAsync();
        foreach (var item in items)
        {
            if (item.Clothes != null)
                clothes.Add(item.Clothes);
            else if (item.Makeup != null)
                makeups.Add(item.Makeup);
            else if (item.Hair != null)
                hairs.Add(item.Hair);
        }
    }

    return (clothes.ToArray(), makeups.ToArray(), hairs.ToArray());
}