many2many_uom_tags

Odoo 19 reworked packaging into plain units of measure, and products gained uom_ids, the list of packagings a product can be sold or moved in. Picking the right unit from a bare name list is error-prone, is a Pack of 6 or of 12?, so this widget extends the standard tags field with a smarter dropdown: under each unit's name, a muted second line shows what that unit converts to for this specific product, computed from the UoM factors and rounded with the reference unit's rounding.

The widget resolves which product it is talking about in two ways: on a product form it uses the record itself, and on any other model it follows the many2one named by product_field. It fetches that product's reference unit once, recomputes when the product changes, and multiplies the hints by the quantity in quantity_field when present.

Unit mistakes are among the most expensive data errors in trading and manufacturing companies: a purchase order in the wrong packaging multiplies a delivery by twelve, and nobody notices until the truck arrives. Surfacing the conversion at the exact moment of selection is a cheap, structural fix, the user picking Dozen sees 12 Units before clicking, and a wrong pick becomes visibly wrong.

Because the widget inherits everything from the tags family, taxonomy governance carries over too: core ships the product form with no_quick_create so users cannot invent a unit from typed text, combined with edit_tags so clicking an existing unit pill opens its record for correction. That combination, curated list plus in-place editing, is the right default for units, which are master data pretending to be tags.

The core pattern: packaging units on the product form

<field name="uom_ids"
       widget="many2many_uom_tags"
       options="{'no_quick_create': True, 'edit_tags': True}"
       groups="uom.group_uom"
       force_save="1"/>

On the product view no product_field is needed: the widget detects it is sitting on product.template and uses the record's own reference unit.

On another model, pointing at the product

<field name="allowed_uom_ids"
       widget="many2many_uom_tags"
       options="{'product_field': 'product_id', 'quantity_field': 'quantity'}"/>

The named product field must be a many2one to product.product or product.template; anything else makes the component throw at setup with an explicit error naming the offending field.

The hint line comes from a custom Many2XUomTagsAutocomplete that overrides the search. Instead of name_search, it runs a search_read pulling each unit's factor, relative_factor, relative_uom_id, and parent_path, then builds the hint two ways: units sharing the reference unit's category, detected by comparing the roots of their parent_path, show quantity times relative_factor in their own relative unit, while unrelated units show quantity times factor divided by the reference factor, in the reference unit. Same-family units are then sorted to the top of the dropdown.

Rounding uses the reference unit's own rounding value in 19. This is one of the places the development branch changes: master fetches the Product Unit decimal precision instead and rounds to digits, caching both that lookup and the unit reads through the ORM's new disk cache.

Odoo 20 is expected at Odoo Experience in Brussels, 24 to 26 September 2026. The development branch is unstable, so everything below is provisional and re-verified after release.

The rework is substantial for a one-release-old widget. The base class changes from the color-editable tags variant to the plain Many2ManyTagsField, and the descriptor now spreads the plain tags options while filtering color_field out; with the color-editable base gone, edit_tags and no_edit_color disappear from the inherited set as well, which touches core's own current usage. Rounding switches from the reference unit's rounding to the Product Unit decimal precision, and the product and precision lookups move to cached ORM calls. The two declared options survive unchanged.