...
Feature:
	Property | CrossReference | Operation;

Property:
	name=ValidID ':' type=JvmTypeReference;
	
CrossReference:
	'ref' name=ValidID ':' type=[Entity|QualifiedName]
;
...

...
// for cross references we create a field
CrossReference : {
	if (f.type?.name == null) return;
	members += f.toField(f.name, f.type.fullyQualifiedName.toString(".").typeRef)
}
...

package org.eclipse.xtext.example.domainmodel.scoping;

import java.util.ArrayList;
import java.util.List;
import org.eclipse.xtext.naming.QualifiedName;
import org.eclipse.xtext.scoping.impl.ImportNormalizer;
import org.eclipse.xtext.scoping.impl.ImportedNamespaceAwareLocalScopeProvider;

public class DomainmodelImportedNamespaceAwareLocalScopeProvider extends ImportedNamespaceAwareLocalScopeProvider 
{
    static final ImportNormalizer modelImport1 = new ImportNormalizer(QualifiedName.create("com","example","domain","model","ordering"), true, false);
    static final ImportNormalizer modelImport2 = new ImportNormalizer(QualifiedName.create("java","lang"), true, false);
    static List<ImportNormalizer> modelImportList = new ArrayList<ImportNormalizer>();
    static 
    {
	    modelImportList.add(modelImport1);
	    modelImportList.add(modelImport2);
    }

    @Override
	  protected List<ImportNormalizer> getImplicitImports(boolean ignoreCase) {
	    return modelImportList;
	  }
}

	@Override
	public void configureIScopeProviderDelegate(com.google.inject.Binder binder) {
		binder.bind(org.eclipse.xtext.scoping.IScopeProvider.class)
			.annotatedWith(Names.named(org.eclipse.xtext.scoping.impl.AbstractDeclarativeScopeProvider.NAMED_DELEGATE))
			.to(org.eclipse.xtext.example.domainmodel.scoping.DomainmodelImportedNamespaceAwareLocalScopeProvider.class);
	}

package com.example.domain.model.ordering {
	
	entity Customer {
		name: String
	}
}

package com.example.domain.model.customer_management {
	
	entity Employee {
		name : String
		ref responsible_customer : Customer
	}
}

package com.example.domain.model.customer_management;

import com.example.domain.model.ordering.Customer;
import org.eclipse.xtext.xbase.lib.Procedures.Procedure1;
import org.eclipse.xtext.xbase.lib.util.ToStringBuilder;

@SuppressWarnings("all")
public class Employee {
  public Employee() {
  }
  
  public Employee(final Procedure1<Employee> initializer) {
    initializer.apply(this);
  }
  
  private String name;
  
  public String getName() {
    return this.name;
  }
  
  public void setName(final String name) {
    this.name = name;
  }
  
  private Customer responsible_customer;
  
  @Override
  public String toString() {
    String result = new ToStringBuilder(this).addAllFields().toString();
    return result;
  }
}

import java.util.AbstractList.ListItr

package A {
	entity X {
		listItr : ListItr
	}
}

import com.example.domain.model.ordering.Customer
	
package com.example.domain.model.customer_management {

	entity Employee {
		name : String
		ref responsible_customer : Customer
	}
}

	override completeCrossReference_Type(EObject model, Assignment assignment, ContentAssistContext context,ICompletionProposalAcceptor acceptor) {
		var crossReference = assignment.getTerminal() as CrossReference
		lookupCrossReference(crossReference, context, acceptor)
	}

@FinalFieldsConstructor
class DomainmodelUiModule extends AbstractModelUiModule {
	def Class<? extends XbaseProposalProvider> bindXbaseProposalProvider() {
		return DomainmodelProposalProvider
	}
	override Class<? extends ITypesProposalProvider> bindITypesProposalProvider() {
		return DomainmodelImportingTypesProposalProvider
	} 	
}

class DomainmodelProposalProvider extends AbstractDomainmodelProposalProvider {

	@Inject
	private RewritableImportSection.Factory importSectionFactory;

	@Inject
	private ReplaceConverter replaceConverter;

	@Inject
	private IScopeProvider scopeProvider;

	def IReplacementTextApplier createTextApplier(ContentAssistContext context, IScope typeScope,
		IQualifiedNameConverter qualifiedNameConverter, IValueConverter<String> valueConverter) {
		if (EcoreUtil2.getContainerOfType(context.getCurrentModel(), XImportSection) !== null)
			return null;
		return new FQNImporter(context.getResource(), context.getViewer(), typeScope, qualifiedNameConverter,
			valueConverter, importSectionFactory, replaceConverter);
	}

	override protected lookupCrossReference(CrossReference crossReference, EReference reference,
		ContentAssistContext contentAssistContext, ICompletionProposalAcceptor acceptor,
		Predicate<IEObjectDescription> filter) {
			super.lookupCrossReference(crossReference, reference, contentAssistContext,
				new ICompletionProposalAcceptor() {

					override accept(ICompletionProposal proposal) {
						if (proposal instanceof ConfigurableCompletionProposal) {
							proposal.textApplier = createTextApplier(
								contentAssistContext,
								scopeProvider.getScope(contentAssistContext.currentModel, reference),
								qualifiedNameConverter,
								qualifiedNameValueConverter
							)
						}
						acceptor.accept(proposal)
					}

					override canAcceptMoreProposals() {
						acceptor.canAcceptMoreProposals()
					}

				}, filter)
		}

	}

grammar org.eclipse.xtext.example.domainmodel.Domainmodel with org.eclipse.xtext.xbase.Xbase
generate domainmodel "http://www.xtext.org/example/Domainmodel"

DomainModel:
	importSection=XImportSection?
	elements+=AbstractElement*;
AbstractElement:
	PackageDeclaration | Entity;
PackageDeclaration:
	'package' name=QualifiedName '{'
		elements+=AbstractElement*
	'}';
Entity:
	'entity' name=ValidID ('extends' superType=JvmTypeReference)? '{'
		features+=Feature*
	'}';
Feature:
	Property | CrossReference | Operation;
Property:
	name=ValidID ':' type=JvmTypeReference;	
CrossReference:
	'ref' name=ValidID ':' type=[Entity|QualifiedName]
;
Operation:
	'op' name=ValidID '(' (params+=FullJvmFormalParameter (',' params+=FullJvmFormalParameter)*)? ')' (':' type=JvmTypeReference)? 
		body=XBlockExpression;