Fix HasOneSql imported field update support

[mvorisek]

fixes #929
closes #933
fixes atk4/filestore#26

data/src/Reference/HasOneSql.php

Lines 35 to 50 in 6657685

	$this->onHookToOurModel($ourModel, Model::HOOK_BEFORE_SAVE, function (Model $ourModel) use ($fieldName, $theirFieldIsTitle, $theirFieldName) {
	// if field is changed, but reference ID field (our_field)
	// is not changed, then update reference ID field value
	if ($ourModel->isDirty($fieldName) && !$ourModel->isDirty($this->our_field)) {
	$theirModel = $this->createTheirModel();
	if ($theirFieldIsTitle) {
	$theirFieldName = $theirModel->title_field;
	}
	$theirModel->addCondition($theirFieldName, $ourModel->get($fieldName));
	$ourModel->set($this->getOurFieldName(), $theirModel->loadOne()->getId());
	if (!$theirFieldIsTitle) { // why for non-title only?
	$ourModel->_unset($fieldName);
	}
	}
	}, [], 20);

hook code is simply wrong.

In 5d49235 the code was intended to be fixed for changed values for different than our_field field. This caused data corruptions like atk4/filestore#26. The data corruption was hotfixed in #1037 but looking into this issue deeper, the hook should simply not be there. We cannot infer our_field values based on "imported"/"joined" fields, as these fields might not be unique and might represent totaly different relation.

The proposed fix maintains the feature for the setting the our_field value for a new record with our field equal to null, under this state, we can determine the foreign/their record based on the imported/joined field without the possibility to mismatch the our field. The relation to foreign/their record can still imply duplicates, but this is a decision by the developer.

Making the field read only after the record is saved will fix also #929.

The update issue is obvious in https://github.com/atk4/data/blob/3.1.0/tests/ReferenceSqlTest.php#L562 test. How on Earth can "first name" solely uniquely locate a foreign user record?