You’re the one giving the database design, so I cannot say whether this is true or false; I simply take at your word that it is true.
Think about that statement from the database point of view though.
How do you put a many-to-many relationship into a database?
Lets say we’ve got post ID 999, and media ID 876.
post ID 999 occupies a single row in the posts table; media ID 876 occupies a single row in the media table.
A single blog post can have many media references. Well, you cant put that information into the post table, because i’ve got 12 media ID’s for this specific post, and I would have 1 media ID field. You cant put that information into the media table, because i’ve got 15 post IDs for this specific piece of media, and 1 post ID field.
That’s why join tables become necessary. A join table has a set of rows, tied to one ID from each table.
A single post can have many matching rows in the join table; but each row of the join table matches to exactly one post.
That same row matches to exactly one media; the media can match many rows in the join table.
In this way, you have shattered the many-to-many relationship between post and media, into two one-to-many relationships - one between the post table and the join table, and one between the media table and the join table.
The word we’re dancing around here is “cardinality”, for the record. When looking at your lines, the cardinality of the line matters, and depends on your origin point. If you start drawing the line at table1, you ask yourself “One row in this table leads to how many entries in the other?”, and draw at the receiving end of the line, the correct symbol for your answer. (The other half of the symbol, “modality”, is basically the minimum number of relations - generally 0 or 1.)
So lets go back to our example, and look at post and post_media_join.
If i start my line at post, and ask the question “One row in post can have how many entries in post_media_join?”, my answer is “Zero or Many”. So i draw a crows foot with a O on my line leading into post_media_join.
I then need the symbol for the other way around; so I start at post_media_join, and ask myself the same question. “One row in post_media_join can have how many entries in post?”, my answer is “One and only one” (Because if it were 0, the row wouldn’t exist; the row MUST contain a post_id, so its Modality is 1). So I would draw the double-I notation on the line leading into post.
