How to model diamond like many-to-many relationship in database ERD

Question

Legend:

• PK (Blue): Primary key
• FK (Green): Foreign key
• PFK (Blue): Primary Key and Foreign Key at the same time

How to model a diamond like (if term is correct) relationship? Better to explain using a simplified example:

There is organization , item and tag entities.

My aim is to model:

1. Every tag is unique by itself and belongs to a single organization.
2. Every item is unique by itself and belongs to a single organization.
3. Items have many tags (joined using M2M table) and related tag / item pairs must belong to same organization. (ie item from organization A cannot pair with a tag from organization B)

I diagrammed two alternative solutions, but none of them satisfied me.

Diagram 1 breaks 3rd aim : items and tags are unique by themselves using id as primary key, but there is nothing to stop insert pairs into item_tag which belong to different organization.

Diagram 2 does not break, but bends 1st and 2nd aims : organization_id is added as a Primary and Foreign Key to item and tag tables and item_tag.organization_id column references both. This prevents pairs from different organization. tag.id and item.id columns are part of a unnecessary composite primary key now, because in reality single id column represents uniqueness of the item and tag .

How can I model those requirements correctly?

Answer 1

To enforce referential integrity, you'll have to ...

• include organization_id in all tables
• create logically redundant UNIQUE (or PK) constraints on (organization_id, id) in both tables tag and item
• have multicolumn FK constraints in item_tag matching the columns of those UNIQUE constraints.

If you don't include the organization_id (logically redundantly) there would be nothing to keep you from linking items and tags from different organizations (by mistake).

That would be your diagram 2 . But do you really need data type uuid for tags? bigint or even int should suffice, while being a bit smaller and faster.

Closely related case with code example for PostgreSQL:

• Enforcing constraints “two tables away”