Referencing foreign keys in the same column

Question

I'm designing a bd-scheme for mysql. My db stores 3 kinds of points: a, b or c and a path is composed by n pair of points:

Route = [ (a1 or b1 or c1 ; a2 or b2 or c2), (a2 or b2 or c2 ; a3 or b3 or c3), ...]

create table a_points (
    point_id serial  not null,
    owner_id bigint unsigned not null,
    name  varchar(20) not null,

    primary key (point_id),
    foreign key (owner_id) references othertable (other_id)
    ) engine = InnoDB;


create table b_points (
    point_id serial  not null,
    owner_id bigint unsigned not null,
    name  varchar(20) not null,
    fields  varchar(20) not null,


    primary key (point_id),
    foreign key (owner_id) references othertable (owner_id)

    ) engine = InnoDB;

create table c_points (
    point_id serial  not null,
    name  varchar(20) not null,
    cfields  varchar(20) not null,

    primary key (point_id)
    ) engine = InnoDB;

create table paths (
    path_id serial   not null,
    name  varchar(20) not null,

    primary key (path_id)
    ) engine = InnoDB;

create table point_pairs (
    pair_id  serial  not null,
    path_id bigint  unsigned not null,
    point_from bigint unsigned not null,
    point_to bigint unsigned not null,
    table_from varchar(9) not null,
    table_to varchar(9) not null,

    primary key (pair_id),
    foreign key (path_id) references paths (path_id)
    ) engine = InnoDB;

(*) a pair of point is (m, n) or from m to n

So I'm storing pair of points together with their path's id. My problem is that I had to create two columns identifying the tables' name of m and n. table_from for m and table_to for n. Thus I'd have to use these two columns in my code to know what kind of points are saved in a route (path and point_pairs table). My question: Does MySql provide something for referencing n foreign keys in the same column? I already thought about joining a, b and c point tables but I'd have to add a type column to this new table and my php classes would become useless.

Thanks in advance!

Answer 1

You're using a pattern called Polymorphic Associations, and no, there's no way to do that and use foreign keys to enforce referential integrity.

I suggest you make one common table that a_points, b_points, and c_points reference. Then your point pairs can reference that common table.

a_points -->
b_points -->  common_points  <-- point_pairs
c_points -->

In other words, the way to make Polymorphic Associations work is to reverse the direction of the reference.

Answer 2

Before I try to pretend to be able to give a meaningful answer, can you state to me what the predicate is of your table point-pairs ? (That is, the MEANING of the rows in the table, expressed in a single English sentence that has a placeholder for each column.)

And while you're at it, maybe the other tables as well ?

I ask so because I fail to understand the problem.