felipepo felipepo - 5 months ago 41
Java Question

Neo4j OGM MappingException

I am trying to get the results of a cypher query on OGM.
Basically, I have been following this tutorial: OGM Tutorial.

I have an entity interface (identical to the one on the tutorial):

public abstract class Entity {

private Long id;

public Long getId() {
return id;
}

@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || id == null || getClass() != o.getClass()) return false;

Entity entity = (Entity) o;

if (!id.equals(entity.id)) return false;

return true;
}

@Override
public int hashCode() {
return (id == null) ? -1 : id.hashCode();
}
}


And I have an class called Motif that inherits from it:

@NodeEntity
public class MyClass extends Entity {
private String field;

@Relationship(type="POINTS", direction=Relationship.OUTGOING)
private Set<Motif> next = new HashSet<Motif>();

public MyClass(String field) {
this.field = field;
}

public String getField(){
return this.field;
}

public void setField(String field) {
this.field = field;
}

public Set<Motif> getNext() {
return next;
}

public void setNext(Set<Motif> next) {
this.next = next;
}

}


I am populating the database with no problem and I can perform the cypher queries within the browser as well.

However, when I try to execute a query on my application with this search class:

public class Searcher {
public Searcher() {

}

public void search() {
String query = "MATCH (a:MyClass)-[r:POINTS]->(b:MyClass) WHERE a.field='B315' RETURN b";
Iterable<MyClass> objs = Neo4jSessionFactory.getInstance().getNeo4jSession().query(MyClass.class, query, Collections.EMPTY_MAP);
for(MyClass obj: objs) {
System.out.println(obj.getField());
}
}
}


I get the following error:

Exception in thread "main" org.neo4j.ogm.exception.MappingException: Error mapping GraphModel to instance of <package>.MyClass
at org.neo4j.ogm.context.GraphEntityMapper.mapEntities(GraphEntityMapper.java:145)
at org.neo4j.ogm.context.GraphEntityMapper.map(GraphEntityMapper.java:117)
at org.neo4j.ogm.context.GraphEntityMapper.map(GraphEntityMapper.java:81)
at org.neo4j.ogm.session.delegates.ExecuteQueriesDelegate.executeAndMap(ExecuteQueriesDelegate.java:111)
at org.neo4j.ogm.session.delegates.ExecuteQueriesDelegate.query(ExecuteQueriesDelegate.java:82)
at org.neo4j.ogm.session.Neo4jSession.query(Neo4jSession.java:323)

Caused by: org.neo4j.ogm.exception.MappingException: Unable to instantiate class <package>.MyClass
at org.neo4j.ogm.annotations.EntityFactory.instantiate(EntityFactory.java:137)
at org.neo4j.ogm.annotations.EntityFactory.instantiateObjectFromTaxa(EntityFactory.java:110)
at org.neo4j.ogm.annotations.EntityFactory.newObject(EntityFactory.java:61)
at org.neo4j.ogm.context.GraphEntityMapper.mapNodes(GraphEntityMapper.java:156)
at org.neo4j.ogm.context.GraphEntityMapper.mapEntities(GraphEntityMapper.java:142)
... 7 more
Caused by: java.lang.NoSuchMethodException: <package>.MyClass.<init>()
at java.lang.Class.getConstructor0(Class.java:3082)
at java.lang.Class.getDeclaredConstructor(Class.java:2178)
at org.neo4j.ogm.annotations.EntityFactory.instantiate(EntityFactory.java:133)
... 11 more


So I might be missing something, because apparently my Motif class cannot be mapped even though it's a node entity.

Answer

Motif.java requires a no arg constructor. This is the reason it cannot be instantiated by the OGM.

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