Spring-boot jpa query with @manyToMany SQL syntax error

Question

I am pretty new to programming and have a fairly complex problem. I was hoping someone knows how to fix the following:

I have a spring-boot project, connecting java to a MySQL database through JPA/hibernate. It works as a restful api and I mostly used annotations.

In the MySQL the 'contacts' table is (among other things) linked to 'projects', through a join table ('CONT_PROJ') and linked to 'coces' through a join table 'CONT$COMPETENCECENTER'. (please try to ignore the fact that the primary key is a double, it was already like this in the database I have to work with and can't change)

In the java code, I made a @ManyToMany linked to a HashSet of projects (one person can have multiple projects and several people can be on the same project), whereas people can only be in 1 coce (therefor I used @ManyToOne)

This is my 'Contact' entity:

@Entity
@Table(name = "contacts")
@JsonInclude(Include.NON_DEFAULT)
public class Contact implements Serializable {
private static final long serialVersionUID = 1L;

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private double k_Contact; 

@NotNull
private String name;

// There are several other variables here

@ManyToMany
@JoinTable(
        name = "PROJ_CONT",
        joinColumns = @JoinColumn(name = "K_CONTACT", referencedColumnName="K_CONTACT"),
        inverseJoinColumns = @JoinColumn(name = "K_PROJECT", referencedColumnName="K_PROJECT"))
    private Set<Project> projects = new HashSet<>();

@ManyToOne  
@JoinTable(
        name = "CONT$COMPETENCECENTER",
        joinColumns = @JoinColumn(name = "K_CONTACT", referencedColumnName="K_CONTACT"),
        inverseJoinColumns = @JoinColumn(name = "F_COCE_MAIN", referencedColumnName="K_CONT_MEDE_COCE"))
    private Coce coce; 

public Contact() {
}

public Contact(double k_Contact) {
    this.k_Contact = k_Contact;
}

public Contact(double k_Contact, String name, String firstName, String email1, Byte[] picture) {
    this.k_Contact = k_Contact;
    this.name = name;
    this.firstName = firstName;
    this.email1 = email1;
    this.picture = picture;
}

public Contact(double k_Contact, String name, String firstName, String email1, Byte[] picture, Coce coce, Set<Project> projects) {
    this.k_Contact = k_Contact;
    this.name = name;
    this.firstName = firstName;
    this.email1 = email1;
    this.picture = picture;
    this.coce = coce;
    this.projects = projects;
}
// plus getters and setters
}

The other entities are very basic (at this point the @ManyToMany and @ManyToOne are one-directional. Getters and setters are present.

This is Project:

@Entity
@Table(name = "projects")
@JsonInclude(Include.NON_DEFAULT)
public class Project implements Serializable {
private static final long serialVersionUID = 1L;

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private double k_Project;

@NotNull
private String name;

@NotNull
private LocalDateTime d_Start;

@NotNull
private LocalDateTime d_End;

public Project() {
}

public Project(double K_PROJECT, String NAME) {
    this.k_Project = K_PROJECT;
    this.name = NAME;
}}

And Coce:

@Entity
@Table(name = "LK_CONT_MEDE_COCE")
@JsonInclude(Include.NON_NULL)
public class Coce implements Serializable {
private static final long serialVersionUID = 1L;

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private double k_Cont_Mede_Coce;

@NotNull
private String competence_Cente;

@NotNull
private String unipartners;

public Coce() {
}

public Coce(double k_Cont_Mede_Coce, String competence_Cente, String unipartners) {
    this.k_Cont_Mede_Coce = k_Cont_Mede_Coce;
    this.competence_Cente = competence_Cente;
    this.unipartners = unipartners;
}}

I only want a few variables (this is why I can't do a select all). I also thought I wasn't supposed to write the extensive query with inner joins, so I didn't (and it works for most queries, except List variables).

This is the relevant part of the ContactRepository:

public interface ContactRepository extends JpaRepository<Contact, Double> {
@Query("select c.k_Contact, c.name, c.firstName, c.email1, c.picture, c.coce, c.project from Contact c where c.k_Contact is :id")
Optional<Contact> findContactById(@Param("id") Double id);
}

I get the following error, but only when I try to add c.projects:

MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'as col_6_0_, coce1_.k_Cont_Mede_Coce as k_Cont_M1_3_0_, project3_.k_Project as k' at line 1

When I do a findAll method on contacts, it also gives all the information correctly, including projects (so the @ManyToMany does work). I can't use this though, because performance would be very bad.

I think it's the way I ask for 'projects' (which is a List), because the Coce (single result, @ManyToOne), works perfectly and returns the correct object, without any problems.

I'm hoping there is a simple mistake I'm making in the query.

If there is anything important I should add here, please let me know. Thanks in advance!

EDIT: The application does run, the SQL error only occurs when I try to access the specific data at runtime. As asked, here is the full error, I hadn't added it because it was quite long:

com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'as col_6_0_, coce1_.k_Cont_Mede_Coce as k_Cont_M1_3_0_, project3_.k_Project as k' at line 1 at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_161] at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_161] at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_161] at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_161] at com.mysql.jdbc.Util.handleNewInstance(Util.java:425) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.mysql.jdbc.Util.getInstance(Util.java:408) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:944) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3976) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3912) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2530) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2683) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2486) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1858) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:1966) ~[mysql-connector-java-5.1.46.jar:5.1.46] at com.zaxxer.hikari.pool.ProxyPreparedStatement.executeQuery(ProxyPreparedStatement.java:52) ~[HikariCP-2.7.8.jar:na] at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.executeQuery(HikariProxyPreparedStatement.java) ~[HikariCP-2.7.8.jar:na] at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.extract(ResultSetReturnImpl.java:60) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.Loader.getResultSet(Loader.java:2168) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1931) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1893) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.Loader.doQuery(Loader.java:938) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:341) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.Loader.doList(Loader.java:2692) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.Loader.doList(Loader.java:2675) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2507) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.Loader.list(Loader.java:2502) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.loader.hql.QueryLoader.list(QueryLoader.java:502) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.hql.internal.ast.QueryTranslatorImpl.list(QueryTranslatorImpl.java:392) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.engine.query.spi.HQLQueryPlan.performList(HQLQueryPlan.java:216) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1489) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.query.internal.AbstractProducedQuery.doList(AbstractProducedQuery.java:1445) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.query.internal.AbstractProducedQuery.list(AbstractProducedQuery.java:1414) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.hibernate.query.internal.AbstractProducedQuery.getSingleResult(AbstractProducedQuery.java:1463) ~[hibernate-core-5.2.16.Final.jar:5.2.16.Final] at org.springframework.data.jpa.repository.query.JpaQueryExecution$SingleEntityExecution.doExecute(JpaQueryExecution.java:214) ~[spring-data-jpa-2.0.6.RELEASE.jar:2.0.6.RELEASE] at org.springframework.data.jpa.repository.query.JpaQueryExecution.execute(JpaQueryExecution.java:91) ~[spring-data-jpa-2.0.6.RELEASE.jar:2.0.6.RELEASE] at org.springframework.data.jpa.repository.query.AbstractJpaQuery.doExecute(AbstractJpaQuery.java:136) ~[spring-data-jpa-2.0.6.RELEASE.jar:2.0.6.RELEASE] at org.springframework.data.jpa.repository.query.AbstractJpaQuery.execute(AbstractJpaQuery.java:125) ~[spring-data-jpa-2.0.6.RELEASE.jar:2.0.6.RELEASE] at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.doInvoke(RepositoryFactorySupport.java:590) ~[spring-data-commons-2.0.6.RELEASE.jar:2.0.6.RELEASE] at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.invoke(RepositoryFactorySupport.java:578) ~[spring-data-commons-2.0.6.RELEASE.jar:2.0.6.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor.invoke(DefaultMethodInvokingMethodInterceptor.java:59) ~[spring-data-commons-2.0.6.RELEASE.jar:2.0.6.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:294) ~[spring-tx-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:98) ~[spring-tx-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:139) ~[spring-tx-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:135) ~[spring-data-jpa-2.0.6.RELEASE.jar:2.0.6.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92) ~[spring-aop-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.data.repository.core.support.SurroundingTransactionDetectorMethodInterceptor.invoke(SurroundingTransactionDetectorMethodInterceptor.java:61) ~[spring-data-commons-2.0.6.RELEASE.jar:2.0.6.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:212) ~[spring-aop-5.0.5.RELEASE.jar:5.0.5.RELEASE] at com.sun.proxy.$Proxy104.findContactById(Unknown Source) ~[na:na] at org.unipartners.services.ContactServiceImpl.findContactById(ContactServiceImpl.java:25) ~[classes/:na] at org.unipartners.web.AppController.findContactById(AppController.java:67) ~[classes/:na] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_161] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_161] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_161] at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_161] at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:209) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:102) ~[spring-webmvc-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:877) ~[spring-webmvc-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:783) ~[spring-webmvc-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991) ~[spring-webmvc-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925) ~[spring-webmvc-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:974) ~[spring-webmvc-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:866) ~[spring-webmvc-5.0.5.RELEASE.jar:5.0.5.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:851) ~[spring-webmvc-5.0.5.RELEASE.jar:5.0.5.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.29.jar:8.5.29] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.5.RELEASE.jar:5.0.5.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) ~[tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459) [tomcat-embed-core-8.5.29.jar:8.5.29] at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.29.jar:8.5.29] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_161] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_161] at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.29.jar:8.5.29] at java.lang.Thread.run(Thread.java:748) [na:1.8.0_161]

Answer 1

You can not execute JPQL like c.coce, c.project when they are another object. You can do the following

@Query("SELECT c FROM Contact c LEFT JOIN FETCH c.projects as pj WHERE c.k_Contact is :id")
Optional<Contact> findById(@Param("id") Double id);

Answer 2

try it add to class Project

@ManyToMany(fetch = FetchType.LAZY, mappedBy = "projects")
private Set<Contact> contacts = new HashSet<Contact>();

My working example

@Entity
public class Role {

    @Id
    @Column(name = "id", unique = true, nullable = false)
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(joinColumns = @JoinColumn(name = "role_id"), inverseJoinColumns = @JoinColumn(name = "user_id"))
    private Set<User> users = new HashSet<>();

    [getter / setter]

}


@Entity
public class User {

    @Id
    @Column(name = "id", unique = true, nullable = false)
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, mappedBy = "users")
    @JsonIgnore
    private Set<Role> userRole = new HashSet<>();

    [getter / setter]

}