The best way to specify foreign keys?

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Sergio

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Dec 8, 2007, 4:41:07 PM12/8/07
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You can read in "Agile Web Development with Rails 2nd ed." that
migrations don't provide a database-independent way to specify foreign
key constraints, so we have to execute native DDL statements, like
this of MySQL:
execute "alter table line_items add constraint fk_line_item_products
foreign key (product_id) references products(id)"

Is this still the only way to specify foreign keys restrictions?

Thanks!

Phillip Koebbe

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Dec 8, 2007, 5:23:55 PM12/8/07
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As far as I know, yes. I wrote a small helper function to do it
though (PostgreSQL syntax):

def foreign_key(foreign_table, foreign_column, primary_table,
primary_column = :id)
execute "
alter table #{foreign_table.to_s}
add constraint fk_#{foreign_table.to_s}_#{foreign_column.to_s}
foreign key (#{foreign_column.to_s}) references #
{primary_table.to_s} (#{primary_column.to_s})
"
end

In my migrations, I call it with

foreign_key :orders, :customer_id, :customers

Peace,
Phillip

Xavier Noria

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Dec 8, 2007, 5:30:51 PM12/8/07
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There's foreign_key_migrations:

http://www.redhillonrails.org/

I use it in all projects whose database supports FKs.

-- fxn

Mail from RAMU

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Dec 8, 2007, 9:52:59 PM12/8/07
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Is there any weblink to the book to download for free " Agile Web Development with Rails 2nd ed. "

Phillip Koebbe

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Dec 8, 2007, 10:56:11 PM12/8/07
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On Dec 8, 2007, at 8:52 PM, Mail from RAMU wrote:

> Is there any weblink to the book to download for free " Agile Web
> Development with Rails 2nd ed. "
>

No, it is not available for free. But it is worth every penny you
pay for it. I use it all the time.

Peace,
Phillip

Mail from RAMU

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Dec 8, 2007, 10:57:53 PM12/8/07
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Thanks Phillip I will take the book :)

Katie Kelly

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Dec 9, 2007, 10:57:14 AM12/9/07
to Ruby on Rails: Talk
I also recommend the plugin from http://www.redhillonrails.org/ for
handling foreign keys.

-Katie

Tom Norian

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Dec 9, 2007, 4:04:35 PM12/9/07
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I'm a complete noobie to databases and programming

but what utility is gained from stuff like:

execute "alter table line_items add constraint fk_line_item_products
> foreign key (product_id) references products(id)"

I mean, there is already validation that they keys are non-null without
it right? If you say a table collumn is "not null" you wont be

The language can't read you mind and assign the foreign key for the
right parent object...that needs to be done through code when you create
or edit an object , right?

I'm so clueless I can't even imagine what potential problem the extra
effort is heading off.
--
Posted via http://www.ruby-forum.com/.

Xavier Noria

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Dec 9, 2007, 4:34:04 PM12/9/07
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On Dec 9, 2007, at 10:04 PM, Tom Norian wrote:

> The language can't read you mind and assign the foreign key for the
> right parent object...that needs to be done through code when you
> create
> or edit an object , right?

The plugin foreign_key_migrations is very transparent. It relies on
standard column naming conventions and provides :references for the
rest.

> I'm so clueless I can't even imagine what potential problem the extra
> effort is heading off.

A couple of benefits: FKs help getting your :dependents right, and as
a side-effect you normally (I don't know whether this is universal)
get an index on the FK column that speeds up some queries.

-- fxn

Tom Norian

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Dec 9, 2007, 5:11:35 PM12/9/07
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>> effort is heading off.
>
> A couple of benefits: FKs help getting your :dependents right, and as
> a side-effect you normally (I don't know whether this is universal)
> get an index on the FK column that speeds up some queries.
>
> -- fxn

Ok, thanks I halfway understand that...as it sinks in and a see the
terms I'll get it more in the weeks ahead.

I have a has_many relationship without the extra foreign key code. I
was just going to use the option , :dependent => :destroy to have the
database delete all the child objects, when the parent is deleted. I
guess if I don't specify that a column is serving as a foreign key the
database needs to look at all the records to see if their is a match? I
can see how indexing would be faster, although I kinda thought that the
has_many statement in the Class definition made a join table
itself.(isn't a join table like an index?)

Xavier Noria

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Dec 9, 2007, 5:36:53 PM12/9/07
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On Dec 9, 2007, at 11:11 PM, Tom Norian wrote:

> I have a has_many relationship without the extra foreign key code. I
> was just going to use the option , :dependent => :destroy to have the
> database delete all the child objects, when the parent is deleted. I
> guess if I don't specify that a column is serving as a foreign key the
> database needs to look at all the records to see if their is a match?

The point is: if for example you forget to add :dependent => :destroy
to a has_* declaration that should have one (which you will at some
point, I promise :-) the application or the corresponding test _dies
miserably_ if you try to delete the parent object. That's because the
FK raises an error from the very database, a row with a FK is invalid
if the corresponding foreign record does not exist.

Thus, you increase the chances of discovering that bug.

-- fxn

Phillip Koebbe

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Dec 9, 2007, 5:55:04 PM12/9/07
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>
> A couple of benefits: FKs help getting your :dependents right, and as
> a side-effect you normally (I don't know whether this is universal)
> get an index on the FK column that speeds up some queries.
>
> -- fxn
>

What database do you use that creates an index on a foreign key? The
databases that I'm familiar with, primarily SQL Server and
PostgreSQL, do *not* automatically create an index on a foreign keyed
column. They only do that on primary keys because the unique index
is used in the enforcing of the of the primary key constraint. A
foreign key is merely a *constraint*, meaning that the database is
going to enforce a rule against the population of that column, and in
this particular case, the data going into it must already exist in
the table/column referenced.

Peace,
Phillip

Phillip Koebbe

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Dec 9, 2007, 6:08:56 PM12/9/07
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On Dec 9, 2007, at 3:04 PM, Tom Norian wrote:

>
> I'm a complete noobie to databases and programming
>
> but what utility is gained from stuff like:
>
> execute "alter table line_items add constraint fk_line_item_products
>> foreign key (product_id) references products(id)"
>
> I mean, there is already validation that they keys are non-null
> without
> it right? If you say a table collumn is "not null" you wont be
>

There are varying opinions about what role a database should play in
your application. On one end of the spectrum is the idea that a
database is nothing more than a place to store data and all
constraints and rules should be enforced in the application. At the
other end is the idea that the database should contain as much
business knowledge as possible, with the primary intent being the
protection of the integrity of the data, and with a secondary intent
of isolating the logic in one place. In the first case, each client
that accesses the data must implement the same rules, and any
variation (or gaps) jeopardizes the value of the data. In the
second, the rules are already in place, so new clients don't have to
worry about it.

Representing these various positions are people who tend to be very
passionate about what they do and believe, so you will often find
extremely opinionated conversations. I definitely agree with the idea
that a database application is only as good as the data it
manipulates. And you can't beat a database engine for enforcing
database rules. So I'm am a firm believer in indexing, constraints,
and stored procedures (where appropriate). I have, however, seen
databases get bogged down by developers trying to do things in them
that are better suited to application code. But that's part of the
debate that I try very diligently to stay out of :).

But don't just take my word (or anyone else's) for it. Get some
experience and decide for yourself.

Peace,
Phillip


Xavier Noria

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Dec 9, 2007, 6:09:04 PM12/9/07
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On Dec 9, 2007, at 11:55 PM, Phillip Koebbe wrote:

> What database do you use that creates an index on a foreign key?

It happens in MySQL with InnoDB[*]:

"InnoDB requires indexes on foreign keys and referenced keys so that
foreign key checks can be fast and not require a table scan. The index
on the foreign key is created automatically."

But looks like that is perhaps more exceptional than normal.

-- fxn

[*] http://dev.mysql.com/doc/refman/5.0/en/innodb-foreign-key-constraints.html

Phillip Koebbe

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Dec 9, 2007, 6:21:57 PM12/9/07
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That's interesting. PostgreSQL requires the *referenced* column to
have a unique constraint or be part of a key, but it makes no
requirement on the foreign key. I'm not a database engineer (dirty
guts of the things), but I don't see how having an index on the
foreign key provides any benefit in enforcing the constraint. The
lookup is going to happen in the referenced table, not the foreign
table.

I am not positive on this, but I don't believe SQL Server requires a
key on the referenced column at all. Given a small table, that
wouldn't make much of a difference, but that'd be huge on something
like an order_detail to orders relationship.

Peace,
Phillip

Lionel Bouton

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Dec 9, 2007, 6:36:15 PM12/9/07
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Phillip Koebbe wrote:
> And you can't beat a database engine for enforcing
> database rules.

In fact you pretty can't avoid it unless you don't rely on constraints
being enforced or have huge amounts of dev resources to throw at the
problem.

I believe there is no way of avoiding associations being broken (leaving
invalid references) when you only declare them at the application level
unless you pretty much reimplement the whole transactional and integrity
constraints logic in a distributed manner in the ORM. To keep things
sane you simply have to enforce them at the database level.
One example is when you delete an object in a process while creating a
child for it in another. There is no way which isn't insanely complex
that you can guarantee that the child won't be created with an invalid
parent_id column value unless you use the database to enforce it. The
simplest way of enforcing this is using foreign keys.

If you really really want to, you could use an archive system for the
whole content of the database, never truly deleting objects only marking
them 'archived'. But you'll have to code your whole application to
support this 'archived' status. It can make sense in some situation, in
fact I'm starting a project where I'll have to use this technique for
some object types to enforce some business rules, trust me it doesn't
make things simpler.

Keep things sane: use Rails validations as your first line of defense
with user-friendly errors, then for the messy race conditions let the
database handle them by throwing exceptions (they can be show stoppers
corrupting your whole database, but they are admittedly relatively rare
cases and a simple page reload usually is enough).

Lionel

Lionel Bouton

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Dec 9, 2007, 6:38:41 PM12/9/07
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Phillip Koebbe wrote:
> That's interesting. PostgreSQL requires the *referenced* column to
> have a unique constraint or be part of a key, but it makes no
> requirement on the foreign key. I'm not a database engineer (dirty
> guts of the things), but I don't see how having an index on the
> foreign key provides any benefit in enforcing the constraint. The
> lookup is going to happen in the referenced table, not the foreign
> table.
>

There's now benefit when you create a reference, but... think 'DELETE'
of the referenced row and you will see a speed benefit :-)

Xavier Noria

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Dec 9, 2007, 6:38:42 PM12/9/07
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On Dec 10, 2007, at 12:21 AM, Phillip Koebbe wrote:

> That's interesting. PostgreSQL requires the *referenced* column to
> have a unique constraint or be part of a key, but it makes no
> requirement on the foreign key. I'm not a database engineer (dirty
> guts of the things), but I don't see how having an index on the
> foreign key provides any benefit in enforcing the constraint. The
> lookup is going to happen in the referenced table, not the foreign
> table.

If you INSERT into a child table you need the to check the parent key
exists. An index in the referenced column is good then, right. But if
you DELETE a row in the parent table a check for that key in the child
table is performed, so an index is good there as well. As far as
speeding up the constraint check itself I think an index makes sense
on both ends (at least conceptually, I am not a database guy).

On the other side performance tips mention that an index is
recommended on foreign keys to speed up joins, so I think that's a
fine feature. I guess other databases do not automate index creation
to be more flexible.

-- fxn

Phillip Koebbe

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Dec 9, 2007, 7:20:23 PM12/9/07
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Ah, the DELETE. Didn't think of that. Thanks to you and Lionel for
reminding me.

On the issue of indexing speeding up queries, that's generally true,
but you have to think about which way your queries are going and
where the look ups are going to be. If you have a list of order
types and an order_type_id in the orders table, the index on
order_type_id is only going to speed up queries when you join from
order_types to orders on order_type_id. If you are querying all
orders and displaying their order_type, such as

select orders.number, order_types.name
from orders
inner join order_types
on order_types.id = orders.order_type_id

the index on order_type_id doesn't do you any good as it's the
primary key column of order_types that is getting searched. But if
you want to do a count of all orders for each order type, that index
on order_type_id will be orders (no pun intended) of magnitude faster
than without it.

I was working on a database recently that over 2600 indexes for 600-
ish tables. I never had the opportunity to do any research to see
which ones were really necessary, but I always wondered how many
weren't.

Peace,
Phillip

Tom Norian

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Dec 9, 2007, 10:03:41 PM12/9/07
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Reading this conversation definitely begins to paint some glimmer of
things that can be eventually understood. Thats the good part.

On the other hand, I'm feel like I hearing about database shoals to
wreck an app's database on at every horizon. Sorta scares me and makes
me feel small and vulnerable...wondering, "what sorta delusional spell
might I be under even thinking a layman can create a robust web app that
will reliably store and manipulate user data". Thats the bad part, lol.

I'll listen, follow convention, and hope for the best. Sounds like
convention points to getting that foreign key language into place for
robustness sake even as it creates some initial room for error in
learning to set it up.

At least working on a rails apps beats doing Sudoku puzzles!

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