8. Inheritance and relationship

Summary:

A table may have multiple types. We can use inheritance to define them.

The Application of Object Relationships solves some complex problems between data tables and tables.

Content:

Forum topic table DBO. Topics:

Field name	Field Type	Can be empty	Remarks
Topicid	Int	Not null	Identity/primary key
Topictitle	Varchar (50)	Not null
Topiccontent	Varchar (max)	Not null
Parenttopic	Int	Null	If the post is subject, this field is null; otherwise, it is the topic ID.
Topictype	Tinyint	Not null	0-theme stickers 1-Reply to post

Define the object class parent class: contains some existing fields.

Then define possible child classes.

Then define the features of each class and field.

Use [inheritancemapping (...)]Judgment type

Use [column (isdiscriminator = true)]Identification basis

// Base class

[Table (name = "topics")]

[Inheritancemapping (code = 0, type = typeof (newtopic), isdefault = true)]

[Inheritancemapping (code = 1, type = typeof (reply)]

Public class topic

{

[Column (isprimarykey = true, isdbgenerated = true)]

Public int topicid {Get; set ;}

[Column]

Public String topictitle {Get; set ;}

[Column]

Public String topiccontent {Get; set ;}

[Column (isdiscriminator = true)]

Public int topictype {Get; set ;}

 

}

// New post, topictype = 0, and the topic field parenttopic is null, that is, it does not exist

Public class newtopic: topic

{

Public newtopic ()

{

Base. topictype = 0;

}

}

// Reply to the post. The post type is topictype = 1 and the topic field is the topic ID.

Public class reply: topic

{

Public reply ()

{

Base. topictype = 1;

}

[Column]

Public int parenttopic {Get; set ;}

}

After defining the object class, the following describes how to use it. First, define datacontext:

// Define the datacontext class

Public partial class bbscontext: datacontext

{

Public table <topic> topics;

Public bbscontext (string connection): Base (connection ){}

}

Read the topic class, and then foreach determines the type row by row, and then converts the type:

Bbscontext CTX = newbbscontext ("");

VaR query = from C in CTX. Topics select C;

Foreach (topic t in query)

{

If (T is newtopic)

{

Newtopic = T asnewtopic;

// Newtopic is the newtopic type that can be used.

}

Else if (T is reply)

{

Reply reply = T asreply;

// Reply is the available reply type.

}

}

Or directly use:

Ienumerable <newtopic> TB = (from t inctx. Topics. oftype <newtopic> () Select T). tolist ();

 

Replyrpl = CTX. Topics. oftype <reply> (). Single (reply => reply. topicid = 8 );

Use of Object Relationships:

1. DBO. categories:

Field name	Field Type	Can be empty	Remarks
Categoryid	Int	Not null	Identity/primary key
Categoryname	Varchar (50)	Not null

2. forum Forum table DBO. boards:

Field name	Field Type	Can be empty	Remarks
Boardid	Int	Not null	Identity/primary key
Boardname	Varchar (50)	Not null
Boardcategory	Int	Not null	Categoryid of the category table corresponding to the Forum section

Entity class:

[Table]

Public class categories

{

Public categories ()

{

This. _ boards = newentityset <boards> ();

}

Private entityset <boards> _ boards;

[Association (thiskey = "category", storage = "_ boards")]

Public entityset <boards> boards

{

Get {return this. _ boards ;}

Set {This. _ boards. Assign (value );}

}

 

[Column (isprimarykey = true, isdbgenerated = true)]

Public int categoryid {Get; set ;}

[Column]

Public String categoryname {Get; set ;}

}

[Table]

Public class boards

{

[Association (thiskey = "category", storage = "_ category")]

Public categories category

{

Get {return this. _ category. entity ;}

Set

{

This. _ category. entity = value;

Value. Boards. Add (this );

}

}

Private entityref <categories> _ category;

 

[Column (isprimarykey = true, isdbgenerated = true)]

Public int boardid {Get; set ;}

[Column]

Public String boardname {Get; set ;}

[Column]

Public int boardcategory {Get; set ;}

}

Usage:

Response. Write ("------------- query section ------------- with 1 Category <br/> ");

VaR query1 = fromb in CTX. Boards whereb. Category. categoryid = 1 select B;

Foreach (boardb in query1)

Response. Write (B. boardid + "" + B. boardname + "<br/> ");

Response. Write ("------------- query the category with more than two sections ------------- <br/> ");

VaR query2 = fromC in CTX. boardcategories where c. Boards. Count> 2 selectc;

Foreach (boardcategoryc in query2)

Response. Write (C. categoryid + "" + C. categoryname + "" + C. Boards. Count + "<br/> ");

Usage:

Dataloadoptionsoptions = new dataloadoptions ();

Options. loadwith <boardcategory> (C => C. Boards );

CTX. loadoptions = options;

Response. Write ("------------- query the category with more than two sections ------------- <br/> ");

Varquery2 = from C inctx. boardcategories where c. Boards. Count> 2 select C;

Foreach (boardcategory C inquery2)

Response. Write (C. categoryid + "" + C. categoryname + "" + C. Boards. Count + "<br/> ");

Usage:

Boardcategory dbcat = new boardcategory () {categoryname = "Database "};

Boardoracle = new Board () {boardname = "oracle", Category = dbcat };

CTX. boardcategories. Add (dbcat );

CTX. submitchanges ();

 

Download source code here