Doctrine\ORM\UnitOfWork::doMerge PHP Method

    private function doMerge($entity, array &$visited, $prevManagedCopy = null, $assoc = null)
    {
        $oid = spl_object_hash($entity);
        if (isset($visited[$oid])) {
            return; // Prevent infinite recursion
        }

        $visited[$oid] = $entity; // mark visited

        $class = $this->em->getClassMetadata(get_class($entity));

        // First we assume DETACHED, although it can still be NEW but we can avoid
        // an extra db-roundtrip this way. If it is not MANAGED but has an identity,
        // we need to fetch it from the db anyway in order to merge.
        // MANAGED entities are ignored by the merge operation.
        if ($this->getEntityState($entity, self::STATE_DETACHED) == self::STATE_MANAGED) {
            $managedCopy = $entity;
        } else {
            // Try to look the entity up in the identity map.
            $id = $class->getIdentifierValues($entity);

            // If there is no ID, it is actually NEW.
            if ( ! $id) {
                $managedCopy = $class->newInstance();
                $this->persistNew($class, $managedCopy);
            } else {
                $managedCopy = $this->tryGetById($id, $class->rootEntityName);
                if ($managedCopy) {
                    // We have the entity in-memory already, just make sure its not removed.
                    if ($this->getEntityState($managedCopy) == self::STATE_REMOVED) {
                        throw new InvalidArgumentException('Removed entity detected during merge.'
                                . ' Can not merge with a removed entity.');
                    }
                } else {
                    // We need to fetch the managed copy in order to merge.
                    $managedCopy = $this->em->find($class->name, $id);
                }

                if ($managedCopy === null) {
                    // If the identifier is ASSIGNED, it is NEW, otherwise an error
                    // since the managed entity was not found.
                    if ($class->isIdentifierNatural()) {
                        $managedCopy = $class->newInstance();
                        $class->setIdentifierValues($managedCopy, $id);
                        $this->persistNew($class, $managedCopy);
                    } else {
                        throw new EntityNotFoundException;
                    }
                }
            }

            if ($class->isVersioned) {
                $managedCopyVersion = $class->reflFields[$class->versionField]->getValue($managedCopy);
                $entityVersion = $class->reflFields[$class->versionField]->getValue($entity);
                // Throw exception if versions dont match.
                if ($managedCopyVersion != $entityVersion) {
                    throw OptimisticLockException::lockFailedVersionMissmatch($entityVersion, $managedCopyVersion);
                }
            }

            // Merge state of $entity into existing (managed) entity
            foreach ($class->reflFields as $name => $prop) {
                if ( ! isset($class->associationMappings[$name])) {
                    if ( ! $class->isIdentifier($name)) {
                        $prop->setValue($managedCopy, $prop->getValue($entity));
                    }
                } else {
                    $assoc2 = $class->associationMappings[$name];
                    if ($assoc2['type'] & ClassMetadata::TO_ONE) {
                        $other = $prop->getValue($entity);
                        if ($other === null) {
                            $prop->setValue($managedCopy, null);
                        } else if ($other instanceof Proxy && !$other->__isInitialized__) {
                            // do not merge fields marked lazy that have not been fetched.
                            continue;
                        } else if ( ! $assoc2['isCascadeMerge']) {
                            if ($this->getEntityState($other, self::STATE_DETACHED) == self::STATE_MANAGED) {
                                $prop->setValue($managedCopy, $other);
                            } else {
                                $targetClass = $this->em->getClassMetadata($assoc2['targetEntity']);
                                $id = $targetClass->getIdentifierValues($other);
                                $proxy = $this->em->getProxyFactory()->getProxy($assoc2['targetEntity'], $id);
                                $prop->setValue($managedCopy, $proxy);
                                $this->registerManaged($proxy, $id, array());
                            }
                        }
                    } else {
                        $mergeCol = $prop->getValue($entity);
                        if ($mergeCol instanceof PersistentCollection && !$mergeCol->isInitialized()) {
                            // do not merge fields marked lazy that have not been fetched.
                            // keep the lazy persistent collection of the managed copy.
                            continue;
                        }

                        $managedCol = $prop->getValue($managedCopy);
                        if (!$managedCol) {
                            $managedCol = new PersistentCollection($this->em,
                                    $this->em->getClassMetadata($assoc2['targetEntity']),
                                    new ArrayCollection
                                    );
                            $managedCol->setOwner($managedCopy, $assoc2);
                            $prop->setValue($managedCopy, $managedCol);
                            $this->originalEntityData[$oid][$name] = $managedCol;
                        }
                        if ($assoc2['isCascadeMerge']) {
                            $managedCol->initialize();
                            if (!$managedCol->isEmpty()) {
                                $managedCol->unwrap()->clear();
                                $managedCol->setDirty(true);
                                if ($assoc2['isOwningSide'] && $assoc2['type'] == ClassMetadata::MANY_TO_MANY && $class->isChangeTrackingNotify()) {
                                    $this->scheduleForDirtyCheck($managedCopy);
                                }
                            }
                        }
                    }
                }
                if ($class->isChangeTrackingNotify()) {
                    // Just treat all properties as changed, there is no other choice.
                    $this->propertyChanged($managedCopy, $name, null, $prop->getValue($managedCopy));
                }
            }
            if ($class->isChangeTrackingDeferredExplicit()) {
                $this->scheduleForDirtyCheck($entity);
            }
        }

        if ($prevManagedCopy !== null) {
            $assocField = $assoc['fieldName'];
            $prevClass = $this->em->getClassMetadata(get_class($prevManagedCopy));
            if ($assoc['type'] & ClassMetadata::TO_ONE) {
                $prevClass->reflFields[$assocField]->setValue($prevManagedCopy, $managedCopy);
            } else {
                $prevClass->reflFields[$assocField]->getValue($prevManagedCopy)->add($managedCopy);
                if ($assoc['type'] == ClassMetadata::ONE_TO_MANY) {
                    $class->reflFields[$assoc['mappedBy']]->setValue($managedCopy, $prevManagedCopy);
                }
            }
        }

        // Mark the managed copy visited as well
        $visited[spl_object_hash($managedCopy)] = true;

        $this->cascadeMerge($entity, $managedCopy, $visited);

        return $managedCopy;
    }