Room学习二：表table的详解

前言

Room是对SQLite的封装，官方推荐使用Room。该文档来自google官方文档。该文档里的代码是kotlin。

一、声明一个table，

@Entity
data class User(
    @PrimaryKey var id: Int,
    var firstName: String?,
    var lastName: String?
)

二、详解

1. 主键 ，使用@PrimaryKey注解;

• 每一个表格必须至少定义一个字段为主键
• 如果有多个字段，则如下

@Entity(primaryKeys = arrayOf("firstName", "lastName"))
data class User(
    var firstName: String?,
    var lastName: String?
)

• 如果需要自动分配ids给表格，则如下

 @PrimaryKey(autoGenerate = true)

3. 一般类名默认为表名，自定义表名这样

@Entity(tableName = "users")
data class User (
    // ...
)

注意：在SQLite中表名对大小写不敏感

4. 一般字段名默认为列名，自定义列名这样

@Entity(tableName = "users")
data class User (
    @PrimaryKey var id: Int,
    @ColumnInfo(name = "first_name") var firstName: String?,
    @ColumnInfo(name = "last_name") var lastName: String?
)

5. 忽视一些字段不要存储在数据库中

@Entity
data class User(
    @PrimaryKey var id: Int,
    var firstName: String?,
    var lastName: String?,
    @Ignore var picture: Bitmap?
)

或者这样

open class User {
    var picture: Bitmap? = null
}

@Entity(ignoredColumns = arrayOf("picture"))
data class RemoteUser(
    @PrimaryKey var id: Int,
    var hasVpn: Boolean
) : User()

6. 添加索引indices，增加查询速度

@Entity(indices = arrayOf(Index(value = ["last_name", "address"])))
data class User(
    @PrimaryKey var id: Int,
    var firstName: String?,
    var address: String?,
    @ColumnInfo(name = "last_name") var lastName: String?,
    @Ignore var picture: Bitmap?
)

7.某些字段必须为唯一的，你可以将使用@Index注解的字段的unique属性设为true，如下

@Entity(indices = arrayOf(Index(value = ["first_name", "last_name"],
        unique = true)))
data class User(
    @PrimaryKey var id: Int,
    @ColumnInfo(name = "first_name") var firstName: String?,
    @ColumnInfo(name = "last_name") var lastName: String?,
    @Ignore var picture: Bitmap?

8.多表关联@ForeignKey,如要关联User表：

@Entity(foreignKeys = arrayOf(ForeignKey(
            entity = User::class,
            parentColumns = arrayOf("id"),
            childColumns = arrayOf("user_id"))
       )
)
data class Book(
    @PrimaryKey var bookId: Int,
    var title: String?,
    @ColumnInfo(name = "user_id") var userId: Int
)

9. @Embedded拆解其他类的字段变为表的字段,如下user表包含Address下的字段：

data class Address(
    var street: String?,
    var state: String?,
    var city: String?,
    @ColumnInfo(name = "post_code") var postCode: Int
)

@Entity
data class User(
    @PrimaryKey var id: Int,
    var firstName: String?,
    @Embedded var address: Address?
)

如果两个类有相同字段，可以使用 @Embedded 添加前缀。

  @Embedded(prefix = "XX_") var address: Address?