# Merging two database records¶

Lino applications can now add a “Merge” action to any model. lino.core.merge

A typical use case are duplicate partners: a user realizes that due to human mistake a given company exists twice in their database, and each of them has certain number of events, invoices, contracts or other related things.

Clicking the “Merge” action on a Partner will ask “Merge this into” (select another Partner), and if you confirm, Lino will move all related data from this Partner to the other one, then delete the first one. Or in other words: if you discover a duplicate record, select the one that is “too much” (usually the one with the bigger id), then run the “Merge” action.

Technical challenges:

• this action also merges related objects linked through a generic foreign key.
• Since I wanted a generic solution it took me some time to figure out how to make Lino support ForeignKey to self for the other field of the action parameter dialog.
• “hard references” (e.g. contracts) are never deleted, but unconditionally moved to the target object. But for certain tables this can be disturbing: That’s why “soft references” (those who are lited in allow_cascaded_delete) may optionally be automatically deleted. The corresponding checkbox fields depend of course on the model, so this action is a first example of an automaically generated Layout.

I also used this occasion to make more intensive usage of Django’s Signals, a feature which I have been ignoring for far too long. lino.core.signals

The change also caused a few internal optimizations:

• Lino no longer uses copy.deepcopy to create copies of each action. (I guess I introduced this before BoundAction existed…)
• lino.ui.base.UI is now a new-style class because I override get_patterns and want to call super().
• AttributeError: ‘WSGIRequest’ object has no attribute ‘raw_post_data’ This was of course due to Django ticket #17323. That’s nice and no problem for me since Lino doesn’t need to run on Djangos before 1.4.2.
• When settings.DEBUG was off, then error messages didnt work because I had converted the error templates to Jinja, but Django’s TEMPLATE_LOADERS wasn’t correctly set (and lino.core.web.Loader had never been tested). This caused TemplateSyntaxErrors like “Could not parse the remainder: ‘(‘bootstrap’,’css’,’bootstrap.css’)’ from ‘site.build_media_url(‘bootstrap’,’css’,’bootstrap.css’)’”