How to check if a table is empty?

Rails
1

Hi
In my web application, a table must have only one record. So when
there is no record, user can add a new row to the table, (a add link
is shown) and when a record is already available, user can’t add a new
one, he can just edit or destroy it( after destroying the current one,
no more records exist, so he can add a new one). But I don’t know how
to check a table for emptiness.
The other problem is when there is one record in table, how can I have
a link to its show action(made by scaffolding), while i don’t know the
id. Because if the user destroy the record, its id is not 1 any more.

2

Hello,

For the first part of your problem, you can check emptiness for your
table
Example with
Example.find(:all).empty?
=> boolean

For the second part,
@object = Example.find(:first)
your_path = example_path(@object)

Of course you can probably do it with a better looking method, but
basically
that’s it.

Cyril

2009/7/10 melomane [email protected]

3

Nice approach, thanks
I did it and it works now.

4

You can also assign a variable and check for an empty array:

find_data = model.find(:all)

if find_data == [] # empty array

table is empty

do something

end

5

Another approach to checking for emptyness in case you need it:

Table.count.zero?

6

Älphä Blüë wrote:

You can also assign a variable and check for an empty array:

find_data = model.find(:all)

if find_data == [] # empty array

table is empty

do something

end

But the variable is unnecessary, and in fact, a more concise way of
saying the same thing would be

if Model.find(:all).empty?

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]

7

But the variable is unnecessary, and in fact, a more concise way of
saying the same thing would be

if Model.find(:all).empty?

Agreed. I was just showing that there are more ways of doing things. I
use your approach…

.empty?
.nil?

… two things I use heavily in my projects…

8

On Jul 12, 10:19 am, “Älphä Blüë” [email protected]
wrote:

… two things I use heavily in my projects…

Typically I use something like ‘unless Model.first’, but are any of
these techniques particularly lighter/faster on the db than others?

-eric

9

On Sun, Jul 12, 2009 at 11:28 AM, Frederick C. <
[email protected]> wrote:

if Model.find(:all).empty?
Typically I use something like ‘unless Model.first’, but are any of
these techniques particularly lighter/faster on the db than others?

It won’t make much difference if you expect your table to genuinely
only contain either 0 rows or a very small number of rows. If it might
contain 10s or hundreds or rows it will be way faster to check
Model.first than it would be to check Model.all.empty? (since you’d be
loading instantiating all hundred objects rather than just 1)

Fred

I agree with Fred’s approach because if you’re testing for emptiness,
you
should do the least amount of work to determine it. For example,

Model.first => will create 1 or no objects
Model.all => will create table size of N or no objects

Good luck,

-Conrad

10

Conrad T. wrote:
[…]

I agree with Fred’s approach because if you’re testing for emptiness,
you
should do the least amount of work to determine it.

Excellent point.

For example,

Model.first => will create 1 or no objects
Model.all => will create table size of N or no objects

Better still:

Model.count => will create one Numeric and that’s all!

Good luck,

-Conrad

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]

11

On Jul 13, 2:41 am, Marnen Laibow-Koser <rails-mailing-l…@andreas-
s.net> wrote:

For example,

Model.first => will create 1 or no objects
Model.all => will create table size of N or no objects

Better still:

Model.count => will create one Numeric and that’s all!

In ruby land yes, this is cheaper but not for the database: with
Model.first the database gets just one row and then stops, with
Model.count it has to count the whole table (which can be expensive on
a large table - usually the db will scan an appropriate index)

Fred

12

I think this is the classic scenario to use find_or_create_by_XXXX

No ‘Summer’ tag exists

Tag.find_or_create_by_name(“Summer”) # equal to Tag.create(:name =>
“Summer”)

Now the ‘Summer’ tag does exist

Tag.find_or_create_by_name(“Summer”) # equal to Tag.find_by_name
(“Summer”)
you can see the documentation here

13

On Jul 12, 7:15 pm, Eric [email protected] wrote:

.empty?
.nil?

… two things I use heavily in my projects…

Typically I use something like ‘unless Model.first’, but are any of
these techniques particularly lighter/faster on the db than others?

It won’t make much difference if you expect your table to genuinely
only contain either 0 rows or a very small number of rows. If it might
contain 10s or hundreds or rows it will be way faster to check
Model.first than it would be to check Model.all.empty? (since you’d be
loading instantiating all hundred objects rather than just 1)

Fred