We have a Spring application that connects with no problems to a MongoDB, with @Autowire and all the rest.
Now we also need the app to connect also to an SQL database.
So we crated an @entity class:
@Entity(name = "SqlCarRecord")
@Table(name = "Cars")
final public class SqlCarRecord {
@Id @GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
@Column(name = "name", nullable = false)
private String name;
....
And a @repository interface:
@Repository 
public interface SqlCarsRepository extends JpaRepository<SqlCarRecord, Long> {
...
And a @Configuraion class like the example here https://www.petrikainulainen.net/programming/spring-framework/spring-data-jpa-tutorial-part-one-configuration/
And in the applicationContext we added 
<jpa:repositories base-package="path.to.interface.package" />
In the pom.xml we already have
        <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-beans</artifactId>
        <version>4.3.5.RELEASE</version>
    </dependency>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-context</artifactId>
        <version>4.3.5.RELEASE</version>
    </dependency>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-core</artifactId>
        <version>4.3.5.RELEASE</version>
    </dependency>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-expression</artifactId>
        <version>4.3.5.RELEASE</version>
    </dependency>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-web</artifactId>
        <version>4.3.5.RELEASE</version>
    </dependency>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-webmvc</artifactId>
        <version>4.3.5.RELEASE</version>
    </dependency>
and we added:
        <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
        <version>2.0.0.M3</version>
    </dependency>
    <dependency>
        <groupId>org.eclipse.persistence</groupId>
        <artifactId>javax.persistence</artifactId>
        <version>2.1.0</version>
    </dependency>
    <!-- DataSource (HikariCP) -->
    <dependency>
        <groupId>com.zaxxer</groupId>
        <artifactId>HikariCP</artifactId>
        <version>2.6.2</version>
    </dependency>
    <!-- JPA Provider (Hibernate) -->
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-entitymanager</artifactId>
        <version>5.2.10.Final</version>
    </dependency>
    <!-- Spring Data JPA -->
    <dependency>
        <groupId>org.springframework.data</groupId>
        <artifactId>spring-data-jpa</artifactId>
        <version>1.11.6.RELEASE</version>
    </dependency>
    <!-- adding this to avoid "java.lang.NoClassDefFoundError: org/w3c/dom/ElementTraversal" -->
    <dependency>
        <groupId>xml-apis</groupId>
        <artifactId>xml-apis</artifactId>
        <version>1.4.01</version>
    </dependency>
    <!-- adding this to avoid "ClassNotFoundException: org.springframework.data.util.CloseableIterator" -->
    <dependency>
        <groupId>org.springframework.data</groupId>
        <artifactId>spring-data-commons</artifactId>
        <version>1.13.6.RELEASE</version>
    </dependency>
And in the @Service class we added:
    ....
    @Autowired
    private SqlCarsRepository carsRepository;
The project is built successfully, but when we try to run it, we get this error:
Context initialization failed
org.springframework.beans.factory.BeanCreationException: Error creating bean 
with name 'jpaMappingContext': Invocation of init method failed; nested 
exception is java.lang.IllegalArgumentException: At least one JPA metamodel 
must be present!
Some of the things We tried:
- change the different versions of spring in the pom,
 - we tried to comment some of them,
 - we tried to change the interface to extend CrudRepository,
 - tried to add an empty constructor to the entity and some other things
 
with no luck.
Will appriciate help.
Thanks in advance.