If the comments all have the same structure, why store them in different tables? Create a comments table and link that to each of the other four tables somehow. (I suspect you need a linking table to make the one-to-many relationship work, but that's pretty standard.)

Then all you need to do is use a templating system that allows you to use fragments or components and define a comment component and reuse that in your other templates.

That's pretty much the idea I had, but I was a bit confused about the linking table thing. What do you mean by that, and how would I do it?


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Don't you just need a field called "parent_type" (or whatever) in the "Comments" table? You'd then be able to query all the comments, all the comments associated with one of the four tables, and (in conjuction with "parent_id") all the comments associated with a specific item in one of the four tables.

In SQL, I would generally handle that something like this:

CREATE TABLE entity_ones (
  id INTEGER,   
  -- whatever goes in this table
);

CREATE TABLE entity_twos (
  id INTEGER,
  -- whatever goes in this table
);

CREATE TABLE comments (
  id INTEGER,
  author VARCHAR(128),
  content TEXT,
  -- etc
);

CREATE TABLE entity_one_comments (
  entity_one_id INTEGER NOT NULL REFERENCES entity_ones(id),
  comment_id INTEGER NOT NULL REFERENCES comments(id),
  PRIMARY KEY ( entity_one_id, comment_id )
);

CREATE TABLE entity_two_comments (
  entity_two_id INTEGER NOT NULL REFERENCES entity_twos(id),
  comment_id INTEGER NOT NULL REFERENCES comments(id),
  PRIMARY KEY ( entity_two_id, comment_id )
);
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