具有自动生成键的外键约束，基本问题（InnoDB）

Question

I'm new to foreign key constraints. I will formulate a simple example to explain my situation.

I have a table user and a table entry . In user there is a user.firstEntry which is a foreign key to entry.EntryID . In entry there is a entry.userID which is a foreign key to the user.userID table. These IDs are all auto increment values.

Are cycles like that forbidden? Then I will have to change the design?

I am not able to insert some valid entry into both tables, because the first insert already says that there's a problem with the constraints. Auto commit is off.

What shall I do?

Thanks

Answer 1

Both user and entry need the other to be already created beforehand. and since either cant be created without the other, you will have this problem IF foreign constraints check is on that is.

Whatever I can understand from your question, each user seems to have multiple entries. So your table design could look like Table_User(user_id(pk), user_name etc) and the entry table could be Table_Entry(entry_id(pk), entry_whatever,...,user_id(fk to user table)) As it seems the user is independent but the entries are dependent on users.

Answer 2

A foreign key constraint is supposed to prevent your from adding invalid data into the foreign key column.

In most cases it will check to see if the value actually exists in the specified table. Because you have a cycle in your user and entry table, when you attempt to create a entry it will check to see if the value of entry.userID exists in the user table. It will do the same when you attempt to add a new user, it will check the entry table for the value you entered for user.firstEntry. If both user and entry are new there is no way to link the two because of your cycle. A new entry record needs an existing user and a new user record needs an existing entry. When both tables are empty I don't think you will be able to satisfy the constraint.

I would suggest keeping the foreign key to userID in the entry table (since I'm assuming entries are linked to users) and finding some other way to represent a user's first entry. Maybe an user_entry_history table or something along those lines.

DISCLAIMER - It's been awhile since I messed with Database design.

Answer 3

Bit strange design, but you can do this:

When creating a User, set firstEntry to NULL. Insert an Entry with that user's id. Update Users and set firstEntry to the id of the inserted entry.