How to solve the problem with two foreign keys?

Question

MySQL

There is a table with elements of the form:

elements
-------------------------------
 `element_id `  | `someData`
-------------------------------
       1        |   blabla
 ------------------------------
       2        |  blablaAgain
-------------------------------

There is a table expressing relationships between elements (if any)

relations
------------------------------------------------
 `element_1_id` |  `element_2_id` |  `relation`

As planned, both elements (element_1_id and element_2_id) should be linked by foreign key to the `elements.element_id` field
. But the "muscular guy" said that it was absolutely impossible to do this (two fields by foreign key to one table).
How to be? Thank you.

Answer 1

An auxiliary table is needed to implement the ManyToMany relationship. If the relationship is OneToMany, you can get away with adding a field of type parent_id to the existing table.

Answer 2

I understand that this is M:M. The relation field is related to the connection itself, some additional data.
Two keys and hang:

ALTER TABLE `relations`
  ADD CONSTRAINT `relations_id1_fk` FOREIGN KEY ( `element_1_id`) REFERENCES `elements` (`element_id`),
  ADD CONSTRAINT `relations_id2_fk` FOREIGN KEY ( `element_1_id`) REFERENCES `elements` (`element_id`);