Added a few tests and supporting models

Working prototype

Added inheritance projection mockup

Added subclass + superclass fields in graphQL test

Beautified tests.  Removed hack/mock in model builder to build to specific classes

Cleaned up name resolution

Code cleanup complete

Build passes with core changes

Refactored tests to use common graphQL utils.  Added error test for interface creation

Fixed build issues

Added tests for invalid filter predicates

Fixed GraphQL errors that return 500 error.  Added more tests to inheritanceIT

Added invalid sort test for JSON-API

Updated comments

Fixing build and codacy issues

Fixed NPE in WebApplicationException handling

Added more unit tests

Fixed broken pom

elide-core/src/main/java/com/yahoo/elide/core/DataStoreTransaction.java

@@ -5,6 +5,7 @@
  */
 package com.yahoo.elide.core;
 
+import com.yahoo.elide.core.exceptions.InvalidEntityBodyException;
 import com.yahoo.elide.core.filter.InPredicate;
 import com.yahoo.elide.core.filter.expression.AndFilterExpression;
 import com.yahoo.elide.core.filter.expression.FilterExpression;
@@ -96,7 +97,8 @@ default <T> T createNewObject(Class<T> entityClass) {
         try {
             obj = entityClass.newInstance();
         } catch (java.lang.InstantiationException | IllegalAccessException e) {
-            obj = null;
+            throw new InvalidEntityBodyException("Cannot create an entity model of type: "
+                    + entityClass.getSimpleName());
         }
         return obj;
     }

elide-core/src/main/java/com/yahoo/elide/core/PersistentResource.java

@@ -388,6 +388,7 @@ public boolean updateAttribute(String fieldName, Object newVal) {
                 transaction.setAttribute(obj, Attribute.builder()
                         .name(fieldName)
                         .type(fieldClass)
+                        .parentType(obj.getClass())
                         .argument(Argument.builder()
                                 .name("_UNUSED_")
                                 .value(newVal).build())

elide-core/src/main/java/com/yahoo/elide/jsonapi/EntityProjectionMaker.java

@@ -331,6 +331,7 @@ private Set<Attribute> getSparseAttributes(Class<?> entityClass) {
 
         return Sets.intersection(allAttributes, sparseFieldsForEntity).stream()
                 .map(attributeName -> Attribute.builder()
+                    .parentType(entityClass)
                     .name(attributeName)
                     .type(dictionary.getType(entityClass, attributeName))
                     .build())

elide-core/src/main/java/com/yahoo/elide/request/Attribute.java

@@ -30,12 +30,27 @@ public class Attribute {
     @ToString.Exclude
     private String alias;
 
+    @ToString.Exclude
+    //If null, the parentType is the same as the entity projection to which this attribute belongs.
+    //If not null, this represents the model type where this attribute can be found.
+    private Class<?> parentType;
+
     @Singular
     @ToString.Exclude
     private Set<Argument> arguments;
 
+    private Attribute(@NonNull Class<?> type, @NonNull String name, String alias, Class<?> parentType,
+                      Set<Argument> arguments) {
+        this.type = type;
+        this.parentType = parentType;
+        this.name = name;
+        this.alias = alias == null ? name : alias;
+        this.arguments = arguments;
+    }
+
     private Attribute(@NonNull Class<?> type, @NonNull String name, String alias, Set<Argument> arguments) {
         this.type = type;
+        this.parentType = null;
         this.name = name;
         this.alias = alias == null ? name : alias;
         this.arguments = arguments;

elide-core/src/main/java/com/yahoo/elide/request/EntityProjection.java

@@ -6,6 +6,7 @@
 
 package com.yahoo.elide.request;
 
+import com.yahoo.elide.core.exceptions.BadRequestException;
 import com.yahoo.elide.core.filter.expression.FilterExpression;
 import com.google.common.collect.Sets;
 import lombok.AllArgsConstructor;
@@ -18,8 +19,6 @@
 import java.util.Optional;
 import java.util.Set;
 
-import javax.ws.rs.BadRequestException;
-
 /**
  * Represents a client data request against a subgraph of the entity relationship graph.
  */
@@ -153,6 +152,7 @@ public EntityProjectionBuilder attributes(Set<Attribute> attributes) {
         public EntityProjectionBuilder relationship(String name, EntityProjection projection) {
             return relationship(Relationship.builder()
                     .alias(name)
+                    .parentType(projection.type)
                     .name(name)
                     .projection(projection)
                     .build());
@@ -177,6 +177,7 @@ public EntityProjectionBuilder relationship(Relationship relationship) {
             if (existing != null) {
                 relationships.remove(existing);
                 relationships.add(Relationship.builder()
+                        .parentType(relationship.getParentType())
                         .name(relationshipName)
                         .alias(relationshipAlias)
                         .projection(existing.getProjection().merge(relationship.getProjection()))
@@ -213,6 +214,7 @@ public EntityProjectionBuilder attribute(Attribute attribute) {
                 attributes.remove(existing);
                 attributes.add(Attribute.builder()
                         .type(attribute.getType())
+                        .parentType(attribute.getParentType())
                         .name(attributeName)
                         .alias(attributeAlias)
                         .arguments(Sets.union(attribute.getArguments(), existing.getArguments()))
@@ -242,6 +244,19 @@ public Attribute getAttributeByAlias(String attributeAlias) {
                     .orElse(null);
         }
 
+        /**
+         * Get an relationship by alias.
+         *
+         * @param relationshipAlias alias to refer to a relationship field
+         * @return found attribute or null
+         */
+        public Relationship getRelationshipByAlias(String relationshipAlias) {
+            return relationships.stream()
+                    .filter(relationship -> relationship.getAlias().equals(relationshipAlias))
+                    .findAny()
+                    .orElse(null);
+        }
+
         /**
          * Check whether a field alias is ambiguous.
          *

elide-core/src/main/java/com/yahoo/elide/request/Relationship.java

@@ -17,27 +17,40 @@
 @Builder
 public class Relationship {
 
-    public RelationshipBuilder copyOf() {
-        return Relationship.builder()
-                .alias(alias)
-                .name(name)
-                .projection(projection);
-    }
-
     @NonNull
     private String name;
 
     private String alias;
 
+    //If null, the parentType is the same as the entity projection to which this relationship belongs.
+    //If not null, this represents the model type where this relationship can be found.
+    private Class<?> parentType;
+
     @NonNull
     private EntityProjection projection;
 
     private Relationship(@NonNull String name, String alias, @NonNull EntityProjection projection) {
         this.name = name;
+        this.parentType = null;
+        this.alias = alias == null ? name : alias;
+        this.projection = projection;
+    }
+
+    private Relationship(@NonNull String name, String alias, Class<?> parentType,
+                         @NonNull EntityProjection projection) {
+        this.name = name;
+        this.parentType = parentType;
         this.alias = alias == null ? name : alias;
         this.projection = projection;
     }
 
+    public RelationshipBuilder copyOf() {
+        return Relationship.builder()
+                .alias(alias)
+                .name(name)
+                .projection(projection);
+    }
+
     public Relationship merge(Relationship toMerge) {
         return Relationship.builder()
                 .name(name)