(If you’re using edge Rails and you’d like to help me test the successor to MetaWhere, please have a look at Squeel)

MetaWhere puts the power of Arel predications (comparison methods) in your ActiveRecord condition hashes.

I hate SQL fragments in Rails code. Resorting to where('name LIKE ?', '%something%') is an admission of defeat. It says, “I concede to allow your rigid, 1970’s-era syntax into my elegant Ruby world of object oriented goodness.” While sometimes such concessions are necessary, they should always be a last resort, because once you move away from an abstract representation of your intended query, your query becomes more brittle. You’re now reduced to hacking about with regular expressions, string scans, and the occasional deferred variable interpolation trick (like ‘#{quoted_table_name}’) in order to maintain some semblance of flexibility.

It isn’t that I hate SQL (much). I’m perfectly capable of constructing complex queries from scratch, and did more than my fair share before coming to the Rails world. It’s that I hate the juxtaposition of SQL against Ruby. It’s like seeing your arthritic grandfather hand in hand with some hot, flexible, yoga instructor. Good for him, but sooner or later something’s going to get broken. It’s like a sentence which, tanpa alasan, perubahan ke bahasa lain, then back again (“for no reason, changes to another language” – with thanks to Google Translate, and apologies to native speakers of Indonesian). It just feels wrong. It breaks the spell – the “magic” that adds to programmer joy, and for no good reason.

MetaWhere is a gem that sets out to right that wrong, and give tranquility to you, the Rails coder.

In your Gemfile:

gem "meta_where"  # Last officially released gem
# gem "meta_where", :git => "git://github.com/ernie/meta_where.git" # Track git repo

or, to install as a plugin:

rails plugin install git://github.com/ernie/meta_where.git

You can use MetaWhere in your usual method chain:

Article.where(:title.matches => 'Hello%', :created_at.gt => 3.days.ago)
=> SELECT "articles".* FROM "articles" WHERE ("articles"."title" LIKE 'Hello%')
   AND ("articles"."created_at" > '2010-04-12 18:39:32.592087')

You can also use similar syntax in a conditions hash supplied to ActiveRecord::Base#find:

Article.find(:all,
  :conditions => {
    :title.matches => 'Hello%',
    :created_at.gt => 3.days.ago
  }
)

They also work in named scopes as you would expect.

class Article
  scope :recent, lambda {|v| where(:created_at.gt => v.days.ago)}
end

Article.recent(14).to_sql
=> SELECT "articles".* FROM "articles"
   WHERE ("articles"."created_at" > '2010-04-01 18:54:37.030951')

Additionally, you can use certain operators as shorthand for certain Arel predication methods.

These are disabled by default, but can be enabled by calling MetaWhere.operator_overload! during your app’s initialization process.

These are experimental at this point and subject to change. Keep in mind that if you don’t want to enclose other conditions in {}, you should place operator conditions before any hash conditions.

Article.where(:created_at > 100.days.ago, :title =~ 'Hi%').to_sql
=> SELECT "articles".* FROM "articles"
   WHERE ("articles"."created_at" > '2010-01-05 20:11:44.997446')
   AND ("articles"."title" LIKE 'Hi%')

Operators are:

• >> (equal)

• ^ (not equal)

• + (in array/range)

• • (not in array/range)

• ~ (matching – not a regexp but a string for SQL LIKE) NOTE: This will override 1.9.x “symbol as string” =~ behavior.¶ ↑

• !~ (not matching, only available under Ruby 1.9)

• > (greater than)

• >= (greater than or equal to)

• < (less than)

• <= (less than or equal to)

• (SQL functions – more on those below)

You can use the & and | operators to perform ands and ors within your queries.

With operators:

Article.where((:title =~ 'Hello%') | (:title =~ 'Goodbye%')).to_sql
=> SELECT "articles".* FROM "articles" WHERE (("articles"."title" LIKE 'Hello%'
   OR "articles"."title" LIKE 'Goodbye%'))

That’s kind of annoying, since operator precedence is such that you have to put parentheses around everything. So MetaWhere also supports a substitution-inspired (String#%) syntax.

With “substitutions”:

Article.where(:title.matches % 'Hello%' | :title.matches % 'Goodbye%').to_sql
=> SELECT "articles".* FROM "articles" WHERE (("articles"."title" LIKE 'Hello%'
   OR "articles"."title" LIKE 'Goodbye%'))

With hashes:

Article.where(
  {:created_at.lt => Time.now} & {:created_at.gt => 1.year.ago}
).to_sql
=> SELECT "articles".* FROM "articles" WHERE
   ((("articles"."created_at" < '2010-04-16 00:26:30.629467')
   AND ("articles"."created_at" > '2009-04-16 00:26:30.629526')))

With both hashes and substitutions:

Article.where(
  :title.matches % 'Hello%' &
  {:created_at.lt => Time.now, :created_at.gt => 1.year.ago}
).to_sql
=> SELECT "articles".* FROM  "articles" WHERE (("articles"."title" LIKE 'Hello%' AND
   ("articles"."created_at" < '2010-04-16 01:04:38.023615' AND
    "articles"."created_at" > '2009-04-16 01:04:38.023720')))

With insanity… errr, complex combinations(*):

Article.joins(:comments).where(
  {:title => 'Greetings'} |
  (
    (
      :created_at.gt % 21.days.ago &
      :created_at.lt % 7.days.ago
    ) &
    :body.matches % '%from the past%'
  ) &
  {:comments => [:body =~ '%first post!%']}
).to_sql
=> SELECT "articles".*
   FROM "articles"
     INNER JOIN "comments"
     ON "comments"."article_id" = "articles"."id"
   WHERE
   ((
     "articles"."title" = 'Greetings'
     OR
     (
       (
         (
           "articles"."created_at" > '2010-03-26 05:57:57.924258'
           AND "articles"."created_at" < '2010-04-09 05:57:57.924984'
         )
         AND "articles"."body" LIKE '%from the past%'
       )
       AND "comments"."body" LIKE '%first post!%'
     )
   ))

(*) Formatting added for clarity. I said you could do this, not that you should. :)

You can choose whether to use an inner join (the default) or a left outer join by tacking .outer or .inner to the symbols specified in your joins() call:

Article.joins(:comments => :moderations.outer).to_sql
=> SELECT "articles".* FROM "articles"
   INNER JOIN "comments" ON "comments"."article_id" = "articles"."id"
   LEFT OUTER JOIN "moderations" ON "moderations"."comment_id" = "comments"."id"

You can use SQL functions in your queries:

Manager.joins(:employees.outer).group('managers.id').
        having(:employees => (:count.func(:id) < 3))
=> SELECT "managers".* FROM "managers"
   LEFT OUTER JOIN "employees" ON "employees"."manager_id" = "managers"."id"
   GROUP BY managers.id HAVING count("employees"."id") < 3

If you enable Symbol operators, you can just use :count[:id], instead of calling func as shown above. SQL functions work in the SELECT, WHERE, and HAVING clauses, and can be aliased with as:

Manager.select('managers.*').
        select(:find_in_set[:id, '3,2,1'].as('position'))
=> SELECT managers.*, find_in_set("managers"."id",'3,2,1') AS position
   FROM "managers"

This is one of those things I hope you find so intuitive that you forget it wasn’t built in already.

PredicateBuilder (the part of ActiveRecord responsible for turning your conditions hash into a valid SQL query) will allow you to nest conditions in order to specify a table that the conditions apply to:

Article.joins(:comments).where(:comments => {:body => 'hey'}).to_sql
=> SELECT "articles".* FROM "articles" INNER JOIN "comments"
   ON "comments"."article_id" = "articles"."id"
   WHERE ("comments"."body" = 'hey')

This feels pretty magical at first, but the magic quickly breaks down. Consider an association named :other_comments that is just a condition against comments:

Article.joins(:other_comments).where(:other_comments => {:body => 'hey'}).to_sql
=> ActiveRecord::StatementInvalid: No attribute named `body` exists for table `other_comments`

Ick. This is because the query is being created against tables, and not against associations. You’d need to do…

Article.joins(:other_comments).where(:comments => {:body => 'hey'})

…instead.

With MetaWhere:

Article.joins(:other_comments).where(:other_comments => {:body => 'hey'}).to_sql
=> SELECT "articles".* FROM "articles" INNER JOIN "comments"
   ON "comments"."article_id" = "articles"."id" WHERE (("comments"."body" = 'hey'))

The general idea is that if an association with the name provided exists, MetaWhere will build the conditions against that association’s table as it’s been aliased, before falling back to assuming you’re specifying a table by name. It also handles nested associations:

Article.where(
  :comments => {
    :body => 'yo',
    :moderations => [:value < 0]
  },
  :other_comments => {:body => 'hey'}
).joins(
  {:comments => :moderations},
  :other_comments
).to_sql
=> SELECT "articles".* FROM "articles"
   INNER JOIN "comments" ON "comments"."article_id" = "articles"."id"
   INNER JOIN "moderations" ON "moderations"."comment_id" = "comments"."id"
   INNER JOIN "comments" "other_comments_articles"
     ON "other_comments_articles"."article_id" = "articles"."id"
  WHERE (("comments"."body" = 'yo' AND "moderations"."value" < 0
    AND "other_comments_articles"."body" = 'hey'))

Contrived example, I’ll admit – but I’ll bet you can think of some uses for this.

One of the changes MetaWhere makes to ActiveRecord is to delay “compiling” the where_values into actual Arel predicates until absolutely necessary. This allows for greater flexibility and last-second inference of associations/joins from any hashes supplied. A drawback of this method is that when merging relations, ActiveRecord just assumes that the values being merged are already firmed up against a specific table name and can just be thrown together. This isn’t the case with MetaWhere, and would cause unexpected failures when merging. However, MetaWhere improves on the default ActiveRecord merge functionality in two ways. First, when called with 1 parameter, (as is always the case when using the & alias) MetaWhere will try to determine if an association exists between the two models involved in the merge. If it does, the association name will be used to construct criteria.

Additionally, to cover times when detection is impossible, or the first detected association isn’t the one you wanted, you can call merge with a second parameter, specifying the association to be used during the merge.

This merge functionality allows you to do this…

(Comment.where(:id < 7) & Article.where(:title =~ '%blah%')).to_sql
=> SELECT "comments".* FROM "comments" INNER JOIN "articles"
   ON "articles"."id" = "comments"."article_id"
   WHERE ("comments"."id" < 7) AND ("articles"."title" LIKE '%blah%')"

…or this…

Article.where(:id < 2).merge(Comment.where(:id < 7), :lame_comments).to_sql
=> "SELECT "articles".* FROM "articles"
   INNER JOIN "comments" ON "comments"."article_id" = "articles"."id"
      AND "comments"."body" = 'first post!'
   WHERE ("articles"."id" < 2) AND ("comments"."id" < 7)"

If you are used to doing stuff like Article.order('title asc'), that will still work as you expect. However, if you pass symbols or arrays in to the order method, you can take advantage of intelligent association detection (as with “Intelligent hash condition mapping,” above) and also some convenience methods for ascending and descending sorts.

Article.order(
  :title.desc,
  :comments => [:created_at.asc, :updated_at]
).joins(:comments).to_sql
=> SELECT "articles".* FROM "articles"
   INNER JOIN "comments" ON "comments"."article_id" = "articles"."id"
   ORDER BY  "articles"."title" DESC,
     "comments"."created_at" ASC, "comments"."updated_at"

Polymorphic associations provide great flexibility, but they can sometimes be a bit of a hassle when it comes to querying through a belongs_to association. First, you have to know what type you’re looking for to do a proper join, and then, you’re forced into using a string join in order to make it happen (which would prevent the use of MetaWhere intelligent condition mapping).

MetaWhere allows you to join polymorphic belongs_to associations like this:

Note.joins(:notable.type(Developer)).
     where(:notable.type(Developer) => {:name.matches => 'Ernie%'})
=> SELECT "notes".* FROM "notes"
   INNER JOIN "developers" ON "developers"."id" = "notes"."notable_id"
     AND "notes"."notable_type" = 'Developer'
   WHERE "developers"."name" LIKE 'Ernie%'

Wouldn’t it be nice if you could do something like this?

# Developer belongs_to Company
company = Company.find(123)
Developer.where(:company => company)

# Developer HABTM Projects
projects = [Project.first, Project.last]
Developer.joins(:projects).where(:projects => projects)

# Note belongs_to :notable, :polymorphic => true
dev1 = Developer.first
dev2 = Developer.last
project = Project.first
company = Company.first
Note.where(:notable => [dev1, dev2, project, company]).to_sql
=> SELECT "notes".* FROM "notes" WHERE (((("notes"."notable_id" IN (1, 8)
   AND "notes"."notable_type" = 'Developer') OR ("notes"."notable_id" = 1
   AND "notes"."notable_type" = 'Project')) OR ("notes"."notable_id" = 1
   AND "notes"."notable_type" = 'Company')))

With MetaWhere, you can.

A huge thank you goes to Pratik Naik (lifo) for a dicussion on #rails-contrib about a patch I’d submitted, and his take on a DSL for query conditions, which was the inspiration for this gem.

There are several ways you can help MetaWhere continue to improve.

• Use MetaWhere in your real-world projects and submit bug reports or feature suggestions.

• Better yet, if you’re so inclined, fix the issue yourself and submit a patch! Or you can fork the project on GitHub and send me a pull request (please include tests!)

• If you like MetaWhere, spread the word. More users == more eyes on code == more bugs getting found == more bugs getting fixed (hopefully!)

• Lastly, if MetaWhere has saved you hours of development time on your latest Rails gig, and you’re feeling magnanimous, please consider making a donation to the project. I have spent hours of my personal time coding and supporting MetaWhere, and your donation would go a great way toward justifying that time spent to my loving wife. :)

Copyright © 2010 Ernie Miller. See LICENSE for details.