postgres key is not present in table constraint

Question

When trying to ALTER TABLE in Postgres 9.5 to create foreign key constraint: from product_template.product_brand_id to product_brand.id

ALTER TABLE public.product_template
    ADD CONSTRAINT product_template_product_brand_id_fkey 
    FOREIGN KEY (product_brand_id)
    REFERENCES public.product_brand (id) MATCH SIMPLE
    ON UPDATE NO ACTION
    ON DELETE SET NULL;

Returns error

ERROR:  insert or update on table "product_template" violates foreign key         constraint "product_template_product_brand_id_fkey"
DETAIL:  Key (product_brand_id)=(12) is not present in table "product_brand".
STATEMENT:  ALTER TABLE "product_template" ADD FOREIGN KEY ("product_brand_id") REFERENCES "product_brand" ON DELETE set null

Im confused why postgres is trying to find product_brand.product_brand_id, when the fkey is from product_template.product_brand_id to product_brand.id

Any ideas?

Answer 1

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.

Answer 2

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.