Update LinkVia documentation - specifying direction seems required

[clepelli]

Hello Graham,

First of all, thanks for your work.

I have generated an OrientDB shema thanks to the default shema generator.
In case of "linkvia" relationships, I have found that I must not rely only on the ToVertex / FromVertex annotations in the Edge class, but I must also specify the direction IN/OUT in the LinkVia annotation (wich defaults to OUT).

If I do not, the relatedProperty may be constructed with the wrong adjacentProperty.

see

spring-data-gremlin/spring-data-gremlin-core/src/main/java/org/springframework/data/gremlin/schema/property/GremlinRelatedProperty.java

Lines 71 to 84 in 126efb2

     if (relatedSchema.isEdgeSchema()) { 
  
         for (Object propertyOfRelatedSchema : relatedSchema.getProperties()) { 
             if (propertyOfRelatedSchema instanceof GremlinAdjacentProperty) { 
                 // If the property has the same direction of the given property here it 
                 // means it is the opposite property of the @EntityRelationship 
                 if (((GremlinAdjacentProperty) propertyOfRelatedSchema).getDirection() == direction.opposite()) { 
                     adjacentProperty = (GremlinAdjacentProperty) propertyOfRelatedSchema; 
                     break; 
                 } 
             } 
         } 
     } 
 } 

And then there is trouble here :

spring-data-gremlin/spring-data-gremlin-core/src/main/java/org/springframework/data/gremlin/schema/writer/AbstractSchemaWriter.java

Line 98 in 126efb2

 Object relatedVertex = createVertexClass(relatedProperty.getAdjacentProperty().getRelatedSchema()); 

What happens then is the edge in the database may link from one of the linked vertices to itself instead of the other.

I would advise to remove the default OUT value and / or update the readme.

Best regards,
Clément.