我想从应用程序引擎的数据存储区获取不同的数据。我使用下面的代码从数据库中获取不同的实体，但它在应用程序引擎服务器上抛出类名不同无法解析异常。
端点类中的代码:
public CollectionResponse<UserMaster> searchUser(
@Named("strPrefix1") String strPrefix1,
@Nullable @Named("cursor") String cursorString,
@Nullable @Named("limit") Integer limit) {
EntityManager mgr = null;
Cursor cursor = null;
List<UserMaster> execute = null;
try {
mgr = getEntityManager();
//here if i remove "distinct" and use "*" selector then it works properly but i want to
//fetch distinct records and there for i need "distinct".
Query query = mgr
.createQuery("select distinct from UserMaster f where f.userName >= :strPrefix1 and f.userName < :strPrefix2");
query.setParameter("strPrefix1", strPrefix1);
query.setParameter("strPrefix2", strPrefix1 + "\ufffd");
if (cursorString != null && cursorString != "") {
cursor = Cursor.fromWebSafeString(cursorString);
query.setHint(JPACursorHelper.CURSOR_HINT, cursor);
}
if (limit != null) {
query.setFirstResult(0);
query.setMaxResults(limit);
}
execute = (List<UserMaster>) query.getResultList();
cursor = JPACursorHelper.getCursor(execute);
if (cursor != null)
cursorString = cursor.toWebSafeString();
for (UserMaster obj : execute)
;
} finally {
mgr.close();
}
return CollectionResponse.<UserMaster> builder().setItems(execute)
.setNextPageToken(cursorString).build();
}
异常(exception):
Class name distinct could not be resolved
org.datanucleus.exceptions.NucleusUserException: Class name distinct could not be resolved
at org.datanucleus.query.expression.PrimaryExpression.bind(PrimaryExpression.java:192)
at org.datanucleus.query.compiler.JavaQueryCompiler.compileResult(JavaQueryCompiler.java:657)
at org.datanucleus.query.compiler.JPQLCompiler.compile(JPQLCompiler.java:83)
at org.datanucleus.store.query.AbstractJPQLQuery.compileInternal(AbstractJPQLQuery.java:271)
at org.datanucleus.store.query.Query.setImplicitParameter(Query.java:799)
at org.datanucleus.api.jpa.JPAQuery.setParameter(JPAQuery.java:437)
at org.datanucleus.api.jpa.JPAQuery.setParameter(JPAQuery.java:57)
at com.sampleregistrationapp.UserFeedMasterEndpoint.searchUser(UserFeedMasterEndpoint.java:620)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:45)
at com.google.api.server.spi.SystemService.invokeServiceMethod(SystemService.java:359)
at com.google.api.server.spi.SystemServiceServlet.execute(SystemServiceServlet.java:127)
at com.google.api.server.spi.SystemServiceServlet.doPost(SystemServiceServlet.java:85)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
最佳答案
此时它甚至与 App Engine 无关:您的 JPQL 查询是错误的，它应该是: :
select distinct f from UserMaster f where f.userName >= :strPrefix1 and f.userName < :strPrefix2
在您的查询中，JPA 将不同的关键字视为您要搜索的类名。
编辑:我很难找到有关 JPA 和 JDO 的文档。对于低级数据存储 API，distinct 查询仅适用于投影查询，即仅返回实体的某些索引属性而不是整个实体的查询。即使使用正确的语法，您的查询也很可能无法运行。
https://developers.google.com/appengine/docs/java/javadoc/com/google/appengine/api/datastore/Query#setDistinct(boolean) =>参见setDistinct(boolean unique)
关于java - 在数据存储应用程序引擎上执行 DISTINCT 查询，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/23152335/