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Java Object Document Mapper for MongoDB
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package org.mongodb.morphia.mapping.lazy.proxy;

import org.mongodb.morphia.Datastore;
import org.mongodb.morphia.Key;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

import static java.lang.String.format;

/**
 * A list of proxied elements
 *
 * @param  the type of the proxied items
 */
public class CollectionObjectReference extends AbstractReference implements ProxiedEntityReferenceList {

    private static final long serialVersionUID = 1L;
    private final List> listOfKeys;

    /**
     * Creates a CollectionObjectReference
     *
     * @param type              the collection
     * @param referenceObjClass the Class of the referenced objects
     * @param ignoreMissing     ignore missing referenced documents
     * @param datastore         the Datastore to use when fetching this reference
     */
    public CollectionObjectReference(final Collection type, final Class referenceObjClass, final boolean ignoreMissing,
                                     final Datastore datastore) {

        super(datastore, referenceObjClass, ignoreMissing);

        object = type;
        listOfKeys = new ArrayList>();
    }

    @Override
    //CHECKSTYLE:OFF
    public void __add(final Key key) {
        //CHECKSTYLE:ON
        listOfKeys.add(key);
    }

    @Override
    //CHECKSTYLE:OFF
    public void __addAll(final Collection> keys) {
        //CHECKSTYLE:ON
        listOfKeys.addAll(keys);
    }

    //CHECKSTYLE:OFF
    @Override
    public List> __getKeysAsList() {
        return Collections.unmodifiableList(listOfKeys);
    }

    @Override
    @SuppressWarnings("unchecked")
    protected void beforeWriteObject() {
        if (__isFetched()) {
            syncKeys();
            ((Collection) object).clear();
        }
    }
    //CHECKSTYLE:ON

    @Override
    @SuppressWarnings("unchecked")
    protected synchronized Object fetch() {
        final Collection c = (Collection) object;
        c.clear();

        final int numberOfEntitiesExpected = listOfKeys.size();
        // does not retain order:
        // List retrievedEntities = p.get().getByKeys(referenceObjClass,
        // (List) __getKeysAsList());

        // so we do it the lousy way: FIXME
        final List retrievedEntities = new ArrayList(listOfKeys.size());
        for (final Key k : listOfKeys) {
            T entity = (T) getDatastore().getByKey(referenceObjClass, k);
            if (entity != null) {
                retrievedEntities.add(entity);
            }
        }

        if (!ignoreMissing && (numberOfEntitiesExpected != retrievedEntities.size())) {
            throw new LazyReferenceFetchingException(format("During the lifetime of a proxy of type '%s', some referenced Entities"
                                                                + " of type '%s' have disappeared from the Datastore.",
                                                            c.getClass().getSimpleName(), referenceObjClass.getSimpleName()));
        }

        c.addAll(retrievedEntities);
        return c;
    }

    private void syncKeys() {
        final Datastore ds = getDatastore();

        listOfKeys.clear();
        for (final Object e : ((Collection) object)) {
            listOfKeys.add(ds.getKey(e));
        }
    }
}