mysql - One to one relationship with foreign key to mutiple tables -


please let me start saying, know has been asked many times before , i've studied other questions (and answers) after 2 days of reading questions , amending database can't work want.

at moment have various tables, example customer, supplier, product, banner, etc.

i have table called custom_field allows custom fields created , used against various other tables.

at moment of tables this:

general tables ==============  customer +-------------+---------------+ | customer_id | customer_name | +-------------+---------------+ | 1           | peter         | | 2           | sally         | +-------------+---------------+  banner +-----------+-------------+--------------+ | banner_id | banner_name | banner_width | +-----------+-------------+--------------+ | 1         | easter      | 100px        | | 2         | xmas        | 250px        | +-----------+-------------+--------------+  tables managing custom fields =================================  custom_field +----------+------------+----------------+-----------+ | field_id | field_name | field_label    | item_type | +----------+------------+----------------+-----------+ | 100      | fav_color  | favorite color | customer  | | 101      | border     | border         | banner    | +----------+------------+----------------+-----------+  custom_field_value +----------+----------+---------+-------------+ | value_id | field_id | item_id | field_value | +----------+----------+---------+-------------+ | 1567     | 100      | 1       | red         | | 1568     | 100      | 2       | blue        | | 1569     | 101      | 1       | solid       | | 1570     | 101      | 2       | dotted      | +----------+----------+---------+-------------+

to clarify, item_id refers customer_id, banner_id, or supplier_id, etc. in example above means peter has "favorite color" custom field set red, , sally has "favorite color" custom field set blue.

the easter banner has "border" custom field set solid, , xmas banner has "border" custom field set dotted.

this works fine, except there can no foreign key or referential integrity set between custom_field_value.item_id , customer.customer_id (or banner.banner_id) because item_id's context described item_type field in custom_field table.

i don't want create multiple nullable foreign keys (not sure work anyway) become unmanageable.

i did try creating sub tables, example customer_custom_field, , relate between customer , custom_field, again becomes unmanageable when consider every table potentially have custom fields.

a single field value ever apply single entity table.

as aside want create attachments table managing uploaded attachments particular entity, , again apply customers, suppliers, products , various other tables, it's similar issue.


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