There a field called Reference
Example of how to use it:

fields.Reference(selection='_ select_target_model', string="Source Document")

Reference field is perfectly fit with your requirement. also, it doesn't require to take many2one field in ir.model
https://github.com/odoo/odoo/blob/12.0/odoo/addons/test_new_api/models.py#L274


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Hi,

Actually if you achieved it some how means, How it will work ? It will generate inconsistency of the data.  Suppose if in the first record, you choose the model as res.partner , so right now the co-model of dynamic field you specified will be res.partner, and if you selected a record and saved, suppose the id be 11 of model res.partner.

Later, in the second record, you choose the model as sale.order , so the co-model for the dynamic field will be now sale.order, and if you choose record and saved, let it be with ID 10. Now what happens is that, in the first record, you will have sale.order(11) instead of res.partner(11).

So, I don't think this will work out.

If you really want to implement this and if you have only need to select a few models, make many2one fields of necessary models and try to hide the unwanted many2one field based on the condition.

Thanks