EntityFramework从延迟加载的外键关系中选择特定的列

Question

我有三个非常简单的表连接在一起。其中两个表格各有三列，主要是整数。第三个约有15列，大多是varchar。

这些表是这样的：Spellbooks,Spells,SpellbookSpells。对于Spellbooks中的每个记录，SpellbookSpells中存在零对多记录，其中有一列为Spellbook_Id和Spell_Id。

所以，当我得到Spellbook book = db.Spellbooks.Find(id);，

我得到一个对象谁拥有SpellbookSpells属性，我可以使用有效地得到List<Spell>。当我通过JSON返回数据的方式，它看起来有点像这样：

{ 
    "SpellbookSpells": [ 
    { 
     "Spell": { 
     "SpellbookSpells": [], 
     "Id": 1, 
     "Name": "Abi-Dalzim's Horrid Wilting", 
     "Description": "<p>You draw the moisture from every creature in a 30-foot cube centered on a point you choose within range. Each creature in that area must make a Constitution saving throw. Constructs and undead aren't affected, and plants and water elementals make this saving throw with disadvantage. A creature takes 10d8 necrotic damage on a failed save, or half as much damage on a successful one.You hurl a bubble of acid. Choose one creature within range, or choose two creatures within range that are within 5 feet of each other. A target must succeed on a Dexterity saving throw or take 1d6 acid damage.</p><p>This spells damage increases by 1d6 when you reach 5th Level (2d6), 11th level (3d6) and 17th level (4d6).</p>", 
     "Page": "ee pc 15", 
     "Range": "150 feet", 
     "Components": "V, S, M", 
     "Ritual": false, 
     "Duration": "Instantaneous", 
     "Concentration": false, 
     "CastingTime": "1 action", 
     "Level": 8, 
     "School": "Necromancy", 
     "Classes": "Sorcerer, Wizard", 
     "Archetype": null, 
     "Domains": null, 
     "Oaths": null, 
     "Circles": null 
     }, 
     "id": 1, 
     "spellbook_id": 5, 
     "spell_id": 1 
    }, 
    { 
     "Spell": { 
     "SpellbookSpells": [], 
     "Id": 2, 
     "Name": "Absorb Elements", 
     "Description": "<p>The spell captures some of the incoming energy, lessening its effect on you and storing it for your next melee attack. You have resistance to the triggering damage type until the start of your next turn. Also, the first time you hit with a melee attack on your next turn, the target takes an extra 1d6 damage of the triggering type, and the spell ends.</p>", 
     "Page": "ee pc 15", 
     "Range": "Self", 
     "Components": "S", 
     "Ritual": false, 
     "Duration": "1 round", 
     "Concentration": false, 
     "CastingTime": "1 action", 
     "Level": 1, 
     "School": "Abjuration", 
     "Classes": "Druid, Ranger, Wizard", 
     "Archetype": null, 
     "Domains": null, 
     "Oaths": null, 
     "Circles": null 
     }, 
     "id": 3, 
     "spellbook_id": 5, 
     "spell_id": 2 
    }, 
    { 
     "Spell": { 
     "SpellbookSpells": [], 
     "Id": 4, 
     "Name": "Aganazzar's Scorcher", 
     "Description": "<p>A line of roaring flame 30 feet long and 5 feet wide emanates from you in a direction you choose. Each creature in the line must make a Dexterity saving throw. A creature takes 3d8 fire damage on a failed save, or half as much damage on a successful one.</p>", 
     "Page": "ee pc 15", 
     "Range": "30 feet", 
     "Components": "V, S, M", 
     "Ritual": false, 
     "Duration": "Instantaneous", 
     "Concentration": false, 
     "CastingTime": "1 action", 
     "Level": 2, 
     "School": "Evocation", 
     "Classes": "Sorcerer, Wizard", 
     "Archetype": null, 
     "Domains": null, 
     "Oaths": null, 
     "Circles": null 
     }, 
     "id": 4, 
     "spellbook_id": 5, 
     "spell_id": 4 
    }, 
    { 
     "Spell": { 
     "SpellbookSpells": [], 
     "Id": 6, 
     "Name": "Alarm", 
     "Description": "<p>You set an alarm against unwanted intrusion. Choose a door, a window, or an area within range that is no larger than a 20-foot cube. Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area. When you cast the spell, you can designate creatures that won’t set off the alarm. You also choose whether the alarm is mental or audible.</p><p>A mental alarm alerts you with a ping in your mind if you are within 1 mile of the warded area. This ping awakens you if you are sleeping.</p><p>An audible alarm produces the sound of a hand bell for 10 seconds within 60 feet.</p>", 
     "Page": "phb 211", 
     "Range": "30 feet", 
     "Components": "V, S, M", 
     "Ritual": true, 
     "Duration": "8 hours", 
     "Concentration": false, 
     "CastingTime": "1 minute", 
     "Level": 1, 
     "School": "Abjuration", 
     "Classes": "Ranger, Ritual Caster, Wizard", 
     "Archetype": null, 
     "Domains": null, 
     "Oaths": null, 
     "Circles": null 
     }, 
     "id": 5, 
     "spellbook_id": 5, 
     "spell_id": 6 
    } 
    ], 
    "Id": 5, 
    "Name": "Krud", 
    "UserId": "6922cf7c-6a86-4b7b-89d9-32ebbae43b8f" 
} 

不过，我想的Spell的属性只有Id，Name，Level和Classes

我怎样才能拿Spellbook book = db.Spellbooks.Find(id);，并且只能选择Spells的特定列？

Answer 1

产生你想要的数据基本查询

var spellData = from sbs in db.SpellBookSpells 
       where sbs.SpellBook_Id == id 
       select new 
       { 
        sbs.Id, 
        sbs.SpellBook_Id, 
        sbs.Spell_Id, 
        Spell = new 
        { 
         sbs.Spell_Id, 
         sbs.Spell.Name, 
         sbs.Spell.Level, 
         sbs.Spell.Classes 
        } 
       }; 

现在是你，如果你想序列化这个匿名类型，或DTO的或原来的实体。

如果这是第一个选项，就完成了。随着DTO的你应该DTO的类名添加到查询：

var spellData = from sbs in db.SpellBookSpells 
       where sbs.SpellBook_Id == id 
       select new SpellBookSpellDto 
       { 
        sbs.Id, 
        sbs.SpellBook_Id, 
        sbs.Spell_Id, 
        Spell = new SpellDto 
        { 
         sbs.Spell_Id, 
         sbs.Spell.Name, 
         sbs.Spell.Level, 
         sbs.Spell.Classes 
        } 
       }; 

为了获得原始的实体，你应该做

var spellDataEntities = from sbs in spellData.AsEnumerable() 
         select new SpellBookSpell 
         { 
          sbs.Id, 
          sbs.SpellBook_Id, 
          sbs.Spell_Id, 
          Spell = new Spell 
          { 
           sbs.Spell_Id, 
           sbs.Spell.Name, 
           sbs.Spell.Level, 
           sbs.Spell.Classes 
          } 
         }; 

的AsEnumerable()调用是必要的，因为你不能在EF创建实体类型LINQ查询（IQueryable）直接。

通过使用这些选项之一，您可以在一个查询中加载所有必需的数据而不是一个字节多。通过使用Find您首先加载一个SpellBook对象，您不再真正使用该对象，之后您将懒惰地加载它的SpellbookSpells。即使您只从此集合中选择了有限数量的属性，也会首先加载完整的实体。