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!
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.
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