GreenDao基本使用
我上一篇讲到了GreenDao3.0的基本使用，真是真真的快，而且很好用。我在最近的一个项目中就尝试的使用了，并且我的这个项目是半离线的项目，所以用到数据库的地方还是比较多的。用来还是比较顺手的。所以跟大家分享一下。
因为我的项目需求，所以我在使用的时候涉及到要使用ToMany来关联数据。所以这里就讲讲我对ToMany的用法。我下面列举一个栗子：假设你的数据库目前有两张表：班级表、学生表。
班级表：里面全部是班级信息，目前的字段只有简单的班级ID班级名称。
学生表：这个表里面装的是全校的学生，该表里面有一个字段是班级id,学生名称。
问题：我要查询某个班的所有学生，打印出来。
思路：
（1）没用ToMany之前的原生SQL，我会这样来做。去查询班级表拿到该班级表的要查询的班级的班级id，然后拿着这个id去遍历学生表。只要是该学生对应的班级id跟我们拿的这个id一致就拿出来。就这样cursor走完之后我就拿到所有的数据啦。（我之前就是这样干的。。。勿喷。。。）
（2）有了ToMany之后，上面的1的操作我们可以不用去关心具体，直接拿到我们的班级对象，用对象getStudents就可以拿到我们要的所有学生信息了。
我贴一下我写的表的实体把：
import org.greenrobot.greendao.annotation.Entity;
import org.greenrobot.greendao.annotation.Id;
import org.greenrobot.greendao.annotation.JoinProperty;
import org.greenrobot.greendao.annotation.ToMany;import java.util.List;
import org.greenrobot.greendao.annotation.Generated;
import org.greenrobot.greendao.DaoException;/*** @author ChenYe* created by on 2018/3/22 0022. 09:23**/
@Entity
public class ClassEntity {@Id(autoincrement = true)private Long id;private String classId;private String className;@ToMany(joinProperties = {@JoinProperty(name = "classId", referencedName = "classId")})private List<StudentEntity> students;/** Used to resolve relations */@Generated(hash = 2040040024)private transient DaoSession daoSession;/** Used for active entity operations. */@Generated(hash = 1384035737)private transient ClassEntityDao myDao;@Generated(hash = 1824774184)public ClassEntity(Long id, String classId, String className) {this.id = id;this.classId = classId;this.className = className;}@Generated(hash = 1465610799)public ClassEntity() {}public Long getId() {return this.id;}public void setId(Long id) {this.id = id;}public String getClassId() {return this.classId;}public void setClassId(String classId) {this.classId = classId;}public String getClassName() {return this.className;}public void setClassName(String className) {this.className = className;}/*** To-many relationship, resolved on first access (and after reset).* Changes to to-many relations are not persisted, make changes to the target entity.*/@Generated(hash = 279833155)public List<StudentEntity> getStudents() {if (students == null) {final DaoSession daoSession = this.daoSession;if (daoSession == null) {throw new DaoException("Entity is detached from DAO context");}StudentEntityDao targetDao = daoSession.getStudentEntityDao();List<StudentEntity> studentsNew = targetDao._queryClassEntity_Students(classId);synchronized (this) {if (students == null) {students = studentsNew;}}}return students;}/** Resets a to-many relationship, making the next get call to query for a fresh result. */@Generated(hash = 238993120)public synchronized void resetStudents() {students = null;}/*** Convenient call for {@link org.greenrobot.greendao.AbstractDao#delete(Object)}.* Entity must attached to an entity context.*/@Generated(hash = 128553479)public void delete() {if (myDao == null) {throw new DaoException("Entity is detached from DAO context");}myDao.delete(this);}/*** Convenient call for {@link org.greenrobot.greendao.AbstractDao#refresh(Object)}.* Entity must attached to an entity context.*/@Generated(hash = 1942392019)public void refresh() {if (myDao == null) {throw new DaoException("Entity is detached from DAO context");}myDao.refresh(this);}/*** Convenient call for {@link org.greenrobot.greendao.AbstractDao#update(Object)}.* Entity must attached to an entity context.*/@Generated(hash = 713229351)public void update() {if (myDao == null) {throw new DaoException("Entity is detached from DAO context");}myDao.update(this);}/** called by internal mechanisms, do not call yourself. */@Generated(hash = 1512363421)public void __setDaoSession(DaoSession daoSession) {this.daoSession = daoSession;myDao = daoSession != null ? daoSession.getClassEntityDao() : null;}
}我先把学生的实体也贴出来一起说：
import org.greenrobot.greendao.annotation.Entity;
import org.greenrobot.greendao.annotation.Id;
import org.greenrobot.greendao.annotation.Generated;/*** @author ChenYe* created by on 2018/3/22 0022. 09:46**/@Entity(nameInDb = "student_table")
public class StudentEntity {@Id(autoincrement = true)private Long id;private String name;private String classId;@Generated(hash = 712916299)public StudentEntity(Long id, String name, String classId) {this.id = id;this.name = name;this.classId = classId;}@Generated(hash = 634333355)public StudentEntity() {}public Long getId() {return this.id;}public void setId(Long id) {this.id = id;}public String getName() {return this.name;}public void setName(String name) {this.name = name;}public String getClassId() {return this.classId;}public void setClassId(String classId) {this.classId = classId;}@Overridepublic String toString() {return "StudentEntity{" +"id=" + id +", name='" + name + '\'' +", classId='" + classId + '\'' +'}';}
}
我贴出来的代码里面包含了GreenDao自动生成的代码。。。其实我们要看的重点是ClassEntity里面有注解ToMany的地方：
@ToMany(joinProperties = {@JoinProperty(name = "classId", referencedName = "classId")})这个。。我习惯的命名一样，所以导致你看起来估计费劲，，，上面的第一个classId 是当前类（ClassEntity）的classId,第二个classId 是StudentEntity 的 calssId,这两个命名你可以随便写的，，，意思就是当前表的classId字段作为外键与另一张表的classId关联起来。
我这个ToMany用法，你可以把外键的类型设置为String类型，如果是另一种用法你的外键只能是Long类型的。用法是：
private Long projectId;@ToMany(referencedJoinProperty = "projectId")List<AccessoryDbEntity> accessoryList;