# Model and lazy loading **URL:** \ **Category:** Rails\ **Created:** [May 1, 2007, 1:17am UTC](https://www.ruby-forum.com/t/model-and-lazy-loading/103035 "2007-05-01T01:17:08Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![mario](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mario](https://www.ruby-forum.com/u/mario)\ **Post date:** [May 1, 2007, 1:17am UTC](https://www.ruby-forum.com/t/model-and-lazy-loading/103035/1 "2007-05-01T01:17:08Z")
Is there a concept of having a model attribute loaded only when necessary. For example, I have a model called product with an attribute called file which is stored in the database as a blob. I would like to be able to retrieve a bunch of products as an array without having to load the file BLOB. Only loading it when necessary. Your help is much appreciated. Mario ---
**Author:** ![mario](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mario](https://www.ruby-forum.com/u/mario)\ **Post date:** [May 1, 2007, 4:49am UTC](https://www.ruby-forum.com/t/model-and-lazy-loading/103035/2 "2007-05-01T04:49:19Z")
You could wrap the file into its own object, and have the “product” model refer to the file object using has\_one or has\_many. This way, when you use the product model, the file model will not be loaded due to the default lazy-loading, and when you need the file, you can just go to product.file, which will load the necessary blob. Hope that helps, ~Ian ---
**Author:** ![mario](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mario](https://www.ruby-forum.com/u/mario)\ **Post date:** [May 1, 2007, 10:11am UTC](https://www.ruby-forum.com/t/model-and-lazy-loading/103035/3 "2007-05-01T10:11:38Z")
Hi, > Is there a concept of having a model attribute loaded only when > necessary. For example, I have a model called product with an > attribute called file which is stored in the database as a blob. I > would like to be able to retrieve a bunch of products as an array > without having to load the file BLOB. Only loading it when necessary. > Not sure if my suggestion is the best since it’s not 100% automatic, but > almost 😉 You could use the :select clause of find, so you select only those attributes you want to retrieve always. Then for the rest of the attributes if you try to read them from the object you would have an error, since they weren’t loaded from db. So here comes the trick… In your :select clause don’t forget to include the id as one of the selected fields. That way you could use it to get the rest of the data from db when you need it. Easiest way of doing this would be a find with a select clause for your blob/clob field. obj=MyHeavyModel.find(:first,:select=\>‘id,name,any\_other\_simple\_fields’) puts obj.id puts obj.name puts obj.blob\_field #this would cause an exception since it’s not loaded puts MyHeavyModel.find(obj.id,:select=\>‘blob\_field’).blob\_field #but this would work In case you have several blob/clob fields in your table, maybe you want to reload them all directly. In that case you don’t even have to write a new find, but just invoke reload over your object.The good thing is reload will not take into account the original select clause, but directly use the id to retrieve the complete row/object from database. obj=MyHeavyModel.find(:first,:select=\>‘id,name,any\_other\_simple\_fields’) puts obj.id puts obj.name puts obj.blob\_field #this would cause an exception since it’s not loaded obj.reload #everything gets retrived from db, even not previously selected fields puts obj.blob\_field #now you have here your field Any of those solutions, should work fine, but you can still make it a bit more transparent if the find/reload looks artificial in your code; you could play with method\_missing of your model so if it’s being called for an attribute which is in your “column\_names” list, it can automatically reload/find the object and return the value of the attribute back. If you go with this approach, just take into account method\_missing is already overwritten in AR classes, so alias the old method and don’t forget to call it before executing your part. Regards, Javier R. - * * * Estamos de estreno… si necesitas llevar el control de tus gastos visita [http://www.gastosgem.com](http://www.gastosgem.com) !!Es gratis!! ---
**Author:** ![mario](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mario](https://www.ruby-forum.com/u/mario)\ **Post date:** [May 3, 2007, 5:06am UTC](https://www.ruby-forum.com/t/model-and-lazy-loading/103035/4 "2007-05-03T05:06:45Z")
I have another question. If I do product.save (where product is an instance of my Product model), here is what the mysql adapter for Ruby will do: SQL: INSERT INTO `PRODUCT` COLUMNS(NAME, PRICE, FILE) VALUES(‘PROD1’, 4.56, X’00DEF033423023220’) All is hunky dory for as long as my BLOB is not too big. If the blob is say 8MB (not that big actually),there is a problem: The buffer used by the mysql server(max\_allowed\_packet) to hold the commands must be at least 16MB (two bytes for hexadecimal representation). This is an unecessarily huge number for holding SQL commands. Is there a way to tell ruby to use a host variable in the SQL statement (i.e. VALUES(‘PROD1’, 4.56, :blobvar) instead of the literal format in the example above. I cannot believe this problem has never been seen before? Imagine if the BLOB data I need to store is 100MB? Your help is much appreciated. ---
**Author:** ![mario](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/8e7dd6/32.png) [@mario](https://www.ruby-forum.com/u/mario)\ **Post date:** [May 1, 2007, 2:12pm UTC](https://www.ruby-forum.com/t/model-and-lazy-loading/103035/5 "2007-05-01T14:12:54Z")
On 4/30/07, Ian B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > You could wrap the file into its own object, and have the “product” > model refer to the file object using has\_one or has\_many. This way, > when you use the product model, the file model will not be loaded due > to the default lazy-loading, and when you need the file, you can just > go to product.file, which will load the necessary blob. > > Hope that helps, > ~Ian This is how my attachment\_fu/acts\_as\_attachment plugin was originally written. Does a Product have a blob for an attached file, or does it have a relation to a ProductFile model with its own metadata? – Rick O. [http://lighthouseapp.com](http://lighthouseapp.com) [http://weblog.techno-weenie.net](http://weblog.techno-weenie.net) [http://mephistoblog.com](http://mephistoblog.com)