EF Code-First 学习之旅 Code First Conventions

协定是一系列的默认规则用来自动配置领域中的概念模型

1：类型发现

　　Code-First对包含DBSet属性的类型创建表（包括这些类型的所有引用类型）

　　

public class Student
{
    public Student()
    { 

    }
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public DateTime DateOfBirth { get; set; }
    public byte[]  Photo { get; set; }
    public decimal Height { get; set; }
    public float Weight { get; set; }

    public Teacher Teacher { get; set; }

    public Standard Standard { get; set; }
}

public class Teacher
{
    public Teacher()
    { 

    }
    public int TeacherId { get; set; }
    public string TeacherName { get; set; }
}
    

namespace EF_Code_First_Tutorials
{

    public class SchoolContext: DbContext
    {
        public SchoolContext(): base()
        {

        }

        public DbSet<Student> Students { get; set; }
        public DbSet<Standard> Standards { get; set; }

    }
}

上图所示：即使上下文不包含Teacher的实体集，Code-First也为Teacher创建表

Code-First发现类型的协定：

1）上下文中的DBSet属性类型

2）引用的类型，即使在不同程序集中

3）DBSet属性的子类

主键约定

默认是：属性名是Id或者是类名+id的属性作为主键

主键的类型可以是任何类型，如果主键的数据类型为数字或者是GUID，则被配置为标识列

如果定义主键为其他名字，则会抛出ModelValidationException 异常

public class Standard
{
    public Standard()
    { 

    }
    public int StdId { get; set; }
    public string StandardName { get; set; }

    public IList<Student> Students { get; set; }

    }

Standard定义StdId为主键，EntityFramework会抛出异常

'System.Data.Entity.ModelConfiguration.ModelValidationException' occurred in EntityFramework.dll
EntityType 'Standard' has no key defined. Define the key for this EntityType.

如果要定义StdId为主键，则需要DataAnnotations or Fluent API来配置主键

关系约定

Code First推断两个实体之间的关系用到导航属性

导航属性可以是简单的引用类型或集合

例如在Student中的导航属性为Standard，Standard中的导航属性为ICollection<Student>

因此，Code First自动为Standard和Student建立一对多的关系

在Student表中建立名为Standard_StandardId 的外键

public class Student
{
    public Student()
    { 

    }
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public DateTime DateOfBirth { get; set; }
    public byte[]  Photo { get; set; }
    public decimal Height { get; set; }
    public float Weight { get; set; }

    //Navigation property
    public Standard Standard { get; set; }
}

public class Standard
{
    public Standard()
    { 

    }
    public int StandardId { get; set; }
    public string StandardName { get; set; }

    //Collection navigation property
    public IList<Student> Students { get; set; }

}

因此，Code First默认约定外键的名字为：<导航属性名>_<导航属性的主键>

外键约定

public class Student
{
    public Student()
    { 

    }
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public DateTime DateOfBirth { get; set; }
    public byte[]  Photo { get; set; }
    public decimal Height { get; set; }
    public float Weight { get; set; }

    //Foreign key for Standard
    public int StandardId { get; set; }

    public Standard Standard { get; set; }
}

public class Standard
{
    public Standard()
    { 

    }
    public int StandardId { get; set; }
    public string StandardName { get; set; }

    public IList<Student> Students { get; set; }

    }

上面的例子中，Student类包含外键StandardId（Standard类的主键）,Code First将在Students表中创建StandardId列名代替Standard_StandardId列

上面的外键不可为null，因为int类型不是nullable

如果定义为nullable，则可为null

复杂类型约定

Code-First默认的约定

Default Convention For	Description
Table Name	<实体类名> + 's'  EF will create DB table with entity class name suffixed by 's'
Primary key Name	1) Id  2) <实体类名> + "Id" (忽略大小写) EF will create primary key column for the property named Id or <Entity Class Name> + "Id" (case insensitive)
Foreign key property Name	By default EF will look for foreign key property with the same name as principal entity primary key name.  If foreign key property does not exists then EF will create FK column in Db table with <Dependent Navigation Property Name> + "_" + <Principal Entity Primary Key Property Name>  e.g. EF will create Standard_StandardId foreign key column into Students table if Student entity does not contain foreignkey property for Standard where Standard contains StandardId
Null column	所有的引用类型列为null，基类型属性为nullable
Not Null Column	主键属性为notnull，non-nullable
DB Columns order	表中的列排序与实体中的属性排序是一样的，不过主键会被移到首位
Properties mapping to DB	所有的属性都被映射到列中，使用[NotMapped]特性排除属性或列不进行映射
Cascade delete	Enabled By default for all types of relationships.

C#中的数据类型与表中的数据类型对应关系

C# DataType	Related DB Column DataType	PK Column DataType & Length
int	int	int, Identity column increment by 1
string	nvarchar(Max)	nvarchar(128)
decimal	decimal(18,2)	decimal(18,2)
float	real	real
byte[]	varbinary(Max)	varbinary(128)
datetime	datetime	datetime
bool	bit	bit
byte	tinyint	tinyint
short	smallint	smallint
long	bigint	bigint
double	float	float
char	No mapping	No mapping
sbyte	No mapping  (throws exception)	No mapping
object	No mapping	No mapping