postgres key is not present in table constraint
The error message simply states that there is at least one row in the table product_template
that contains the value 12
in the column product_brand_id
But there is no corresponding row in the table product_brand
where the column id
contains the value 12
Key (product_brand_id)=(12)
relates the source column of the foreign key, not the target column.
In simple terms, the value of FOREIGN KEY(product_brand_id) provided in your ALTER statement is not present in the source (product_brand) table.