[英]Parent child relation in Linq query
In the Entity Class 在实体类中
I have two parent table "TestParent" and "TestTag" in Entity class and one of the child table "TestChild" which is not updated in Entity class. 我在实体类中有两个父表“ TestParent”和“ TestTag”,而在实体类中没有更新的子表“ TestChild”之一。 so can I Join the TestParent and TestChild table (both are foreinkey primary relation).
所以我可以加入TestParent和TestChild表(两者都是foreinkey主要关系)。
Issue: I am not able to select child table TestChild in Linq query 问题:我无法在Linq查询中选择子表TestChild
Table : TestTag: 表:TestTag:
Table: TestParent: 表:TestParent:
Childtable: TestChild 孩童:TestChild
In Entity class below 在下面的实体类中
public DbSet<TestParent> Questions { get; set; }
public DbSet<TestTag> Tags { get; set; }
So you have three tables: a table of TestTags
, a table of TestParents
and a table with TestChildren
. 因此,您具有三个表:
TestTags
表, TestParents
表和带有TestChildren
的表。
Every TestTag
has zero or more TestChildren
; 每个
TestTag
都有零个或多个TestChildren
; every TestChild
belongs to exactly one TestTag
, using a foreign key. 每个
TestChild
使用外键完全属于一个TestTag
。 A straightforware one-to-many relation 直接的一对多关系
Every TestParent
also has zero or more TestChildren
; 每个
TestParent
还具有零个或多个TestChildren
; every TestChild
has exactly one TestParent
, using a foreign key. 每个
TestChild
都有一个使用外键的TestParent
。
If you followed the entity framework code first conventions , you would have something similar to 如果您遵循实体框架代码优先约定 ,那么您将拥有类似于
class TestTag
{
public int Id {get; set;}
... // other properties
// every TestTag has zero or more TestChildren (one-to-many)
public virtual ICollection<TestChild> TestChildren {get; set;}
}
class TestParent
{
public int Id {get; set;}
... // other properties
// every TestParent has zero or more TestChildren (one-to-many)
public virtual ICollection<TestChild> TestChildren {get; set;}
}
class TestChild
{
public int Id {get; set;}
... // other properties
// every TestChild has exactly one TestParent using foreign key
public int TestParentId {get; set;}
public virtual TestParent TestParent {get; set;}
// every TestChild has exactly one TestTag using foreign key
public int TestTagId {get; set;}
public virtual TestTag TestTag {get; set;}
}
In entity framework the columns of the tables are represented by non-virtual properties.
在实体框架中,表的列由非虚拟属性表示。 The virtual properties represent the relations between the tables (one-to-many, many-to-many, ...)
虚拟属性表示表之间的关系(一对多,多对多,...)
This way it is possible for TestTag 4 and TestParent 5 to have two Children. 这样,TestTag 4和TestParent 5可以有两个子代。 If you don't want, consider creating a combined primary key [TestTagId, TestParentId]
如果您不想要,请考虑创建组合的主键[TestTagId,TestParentId]
If TestChild has no other properties than the foreign keys, than in fact you are creating a many-to-many relation between TestTag and TestParent. 如果TestChild除了外键之外没有其他属性,那么实际上您将在TestTag和TestParent之间创建多对多关系。 In that case you don't have to mention the TestChild table.
在这种情况下,您不必提及TestChild表。
For completeness the DbContext: 为了完整起见,DbContext:
class MyDbContext : DbContext
{
public DbSet<TestTag> TestTags {get; set;}
public DbSet<TestParent> TestParents {get; set;}
public DbSet<TestChild> TestChildren {get; set;}
}
This is all that entity framework needs to know to detect your relations and the primary and foreign keys. 这是实体框架检测您的关系以及主键和外键所需要知道的全部内容。 Because I followed the conventions attributes nor fluent API is necessary, except maybe because of the non-standard plural form
Children
. 因为我遵循了约定的属性,所以也不需要流利的API,除非是因为非标准的复数形式
Children
。
so can I Join the TestParent and TestChild table
所以我可以加入TestParent和TestChild表
Well you can do a (group-)join, but it is usually easier to use the ICollections
好了,您可以执行(group-)join,但是使用
ICollections
通常更容易
Give me the TestParents that ... with all their TestChildren and TestTags that ... 给我...的TestParents及其所有的TestChildren和TestTags ...
var result = dbContext.TestParents
.Where(parent => ...) // if you don't want all testParents
.Select(parent => new
{
// select only the properties you plan to use
Id = parent.Id,
Name = parent.Name,
Children = parent.TestChildren
.Where(child => child.TestTag.Name = "My Test Tag Name")
.Select(child => new
{
Name = child.TestTag.Name,
...
})
.ToList(),
});
Some people prefer to do a GroupJoin instead. 有些人喜欢改用GroupJoin。 If you want that, and you can convince your project leader that a GroupJoin is better readable / testable / maintainable than using the ICollections, you can do the following:
如果您希望这样做,并且可以说服项目负责人与使用ICollections相比,GroupJoin具有更好的可读性/可测试性/可维护性,则可以执行以下操作:
var result = dbContext.TestParents.GroupJoin(dbContext.TestChildren,
parent => parent.Id, // from each parent take the Id
child => child.ParentId, // from each child take the ParentId
// resultSelector:
(parent, children) => new
{
// Parent Properties:
Id = parent.Id,
...
// GroupJoin the collection of child properties with the TestTags:
children.GroupJoin(dbContext.TestTags,
child => child.TestTagId, // from each TestChild take the TestTagId
tag => tag.Id, // from each TestTag take the Id
(child, tagsOfThisChild) => new
{
// desired Child properties
...
TestTags = tagsOfThisChild.Select(tag => new
{
Id = tag.Id,
...
})
.ToList(),
})
.ToList(),
});
IMHO: this looks horrible! 恕我直言:这看起来太可怕了!
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