Question

I have a simple site where users can bid on a product. The model looks like this:

Obviously different users can bid, so how do I draw the missing link to the ‘users’ table?

I’m a little confused whether to use identifying or non-identifying relationship.
Updated:

Answer 1

You say: “… users can bid on a product.“

This suggests that there is a relationship (Bids) between Users and Products. You could name it user_bids_on_product or just bid.

I would remove the relationship between user and product (unless it means something else like the owner of a product) and unify your bids and bids_has_product into one table:

user_bids_on_product
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product_id  FK to product
user_id     FK to user
price

The (product_id, user_id) should not be the Primary Key for this table, as we assume a user can bid multiple times on a product.

You could add a surrogate id and make that the PK or add a bid_number column and make the compound (product_id, user_id, bid_number) the PK. You could alternatively make the (product_id, bid_number) the PK (the bid_number could mark the order of the bids per product, like an auction in this case). I think that’s up to you to decide.

Identifying relationships:

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Non-identifying relationships: