# Chaining queries in ActiveRecord

**URL:** <https://discuss.rubyonrails.org/t/chaining-queries-in-activerecord/48615>\
**Category:** rubyonrails-talk\
**Created:** [January 15, 2010, 9:25pm UTC](https://discuss.rubyonrails.org/t/chaining-queries-in-activerecord/48615 "2010-01-15T21:25:49Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![parkurm](https://avatars.discourse-cdn.com/v4/letter/p/90db22/32.png) [@parkurm](https://discuss.rubyonrails.org/u/parkurm)\
**Post date:** [January 15, 2010, 9:25pm UTC](https://discuss.rubyonrails.org/t/chaining-queries-in-activerecord/48615/1 "2010-01-15T21:25:49Z")

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Hi all,

Stuck at this problem for several days. Tried to use named\_scope, scope\_out, and plugin such as searchlogic, but no result.

The problem is as follows: I have a pic table and tag table, and a join table called pic\_tag (with only pic\_id and tag\_id) so that I can associate N pics to M tags. This is the tutorial way to set up a many- to-many association. I'm trying to implement a simple search function, so that I can search for a picture with several given tags.

Now say I want to search for pics that are tagged with "dog" and "cat". The simplest way to do this is to use named\_scopes, for example:

[code] class Pic \< ActiveRecord::Base &nbsp;&nbsp;has\_many :pic\_tags &nbsp;&nbsp;has\_many :tags, :through =\> :pic\_tags

&nbsp;&nbsp;named\_scope :dog, options\_for\_tag(“dog") # options\_for\_tag(tag) is a method to generate joined table search conditions, not important &nbsp;&nbsp;named\_scope :cat, options\_for\_tag("cat") end [/code]

But when I chain the query like this: [code]Pic.dog.cat[/code] The SQL query that I'm actually getting is the merged conditions: [code] SELECT "pics".\* FROM "pics" INNER JOIN "pic\_tags" ON ("pics"."id" = "pic\_tags"."pic\_id") INNER JOIN "tags" ON ("tags"."id" = "pic\_tags"."tag\_id") WHERE ((tags.name LIKE 'dog') AND (tags.name LIKE 'cat')) [/code] And the above query won't give me any result, since no joined table entry will have both 'dog' and 'cat' at the same time. The reason for this is that using named\_scope, query is done only when all the query conditions are merged together. I tried scope\_out, searchlogic plugin, but they all share the same basic concept of using named\_scope, so the result is the same.

So here's my question: is there any way to query step by step? If so, then Pic.dog should be able to return all pics tagged with "dog", and then when I continue to query on the result, say, Pic.dog.cat, I can have all pics tagged with "cat" based on the previous result.

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**Author:** ![Matt\_Jones](https://avatars.discourse-cdn.com/v4/letter/m/258eb7/32.png) [@Matt\_Jones](https://discuss.rubyonrails.org/u/Matt_Jones)\
**Post date:** [January 16, 2010, 7:35pm UTC](https://discuss.rubyonrails.org/t/chaining-queries-in-activerecord/48615/2 "2010-01-16T19:35:07Z")

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The classical way to handle this is to alias the tables involved so they don't get fused together by the SQL generation code.

For instance:

named\_scope :dog, :joins =\> 'INNER JOIN pic\_tags AS dog\_pic\_tags ON (pics.id = dog\_pic\_tags.pic\_id) INNER JOIN tags AS dog\_tags ON (dog\_tags.id = dog\_pic\_tags.tag\_id)', :conditions =\> ['dog\_tags.name LIKE ?', 'dog']

This gives each scope its own set of distinct joins, so it should avoid the previous problem. It's straightforward to extend this to a named\_scope named tagged\_with:

named\_scope :tagged\_with, lambda { |n| { :joins =\> "INNER JOIN pic\_tags AS #{n}\_pic\_tags ON (pics.id = #{n}\_pic\_tags.pic\_id) INNER JOIN tags AS #{n}\_tags ON (#{n}\_tags.id = #{n} \_pic\_tags.tag\_id)", :conditions =\> ["#{n}\_tags.name LIKE ?',n] } }

Then you can do things like:

Pic.tagged\_with('dog').tagged\_with('cat') etc.

Note that you'll want to sanitize any user inputs you're passing to tagged\_with.

--Matt Jones