Paginating finders for ActiveRecord models
WillPaginate adds paginate, per_page and other methods to ActiveRecord::Base class methods and associations. It also hooks into method_missing to intercept pagination calls to dynamic finders such as paginate_by_user_id and translate them to ordinary finders (find_all_by_user_id in this case).
In short, paginating finders are equivalent to ActiveRecord finders; the only difference is that we start with “paginate” instead of “find” and that :page is required parameter:
@posts = Post.paginate :all, :page => params[:page], :order => 'created_at DESC'
In paginating finders, “all” is implicit. There is no sense in paginating a single record, right? So, you can drop the :all argument:
Post.paginate(...) => Post.find :all Post.paginate_all_by_something => Post.find_all_by_something Post.paginate_by_something => Post.find_all_by_something
The importance of the :order parameter
In ActiveRecord finders, :order parameter specifies columns for the ORDER BY clause in SQL. It is important to have it, since pagination only makes sense with ordered sets. Without the ORDER BY clause, databases aren’t required to do consistent ordering when performing SELECT queries; this is especially true for PostgreSQL.
Therefore, make sure you are doing ordering on a column that makes the most sense in the current context. Make that obvious to the user, also. For perfomance reasons you will also want to add an index to that column.
Public instance methods
This is the main paginating finder.
Special parameters for paginating finders
- :page — REQUIRED, but defaults to 1 if false or nil
- :per_page — defaults to CurrentModel.per_page (which is 30 if not overridden)
- :total_entries — use only if you manually count total entries
- :count — additional options that are passed on to count
- :finder — name of the ActiveRecord finder used (default: “find”)
All other options (conditions, order, …) are forwarded to find and count calls.
# File lib/will_paginate/finder.rb, line 64 def paginate(*args) options = args.pop page, per_page, total_entries = wp_parse_options(options) finder = (options[:finder] || 'find').to_s if finder == 'find' # an array of IDs may have been given: total_entries ||= (Array === args.first and args.first.size) # :all is implicit args.unshift(:all) if args.empty? end WillPaginate::Collection.create(page, per_page, total_entries) do |pager| count_options = options.except :page, :per_page, :total_entries, :finder find_options = count_options.except(:count).update(:offset => pager.offset, :limit => pager.per_page) args << find_options # @options_from_last_find = nil pager.replace(send(finder, *args) { |*a| yield(*a) if block_given? }) # magic counting for user convenience: pager.total_entries = wp_count(count_options, args, finder) unless pager.total_entries end end
Wraps find_by_sql by simply adding LIMIT and OFFSET to your SQL string based on the params otherwise used by paginating finds: page and per_page.
Example:
@developers = Developer.paginate_by_sql ['select * from developers where salary > ?', 80000],
:page => params[:page], :per_page => 3
A query for counting rows will automatically be generated if you don’t supply :total_entries. If you experience problems with this generated SQL, you might want to perform the count manually in your application.
# File lib/will_paginate/finder.rb, line 131 def paginate_by_sql(sql, options) WillPaginate::Collection.create(*wp_parse_options(options)) do |pager| query = sanitize_sql(sql.dup) original_query = query.dup # add limit, offset add_limit! query, :offset => pager.offset, :limit => pager.per_page # perfom the find pager.replace find_by_sql(query) unless pager.total_entries count_query = original_query.sub /\bORDER\s+BY\s+[\w`,\s]+$/mi, '' count_query = "SELECT COUNT(*) FROM (#{count_query})" unless self.connection.adapter_name =~ /^(oracle|oci$)/i count_query << ' AS count_table' end # perform the count query pager.total_entries = count_by_sql(count_query) end end end
Iterates through all records by loading one page at a time. This is useful for migrations or any other use case where you don’t want to load all the records in memory at once.
It uses paginate internally; therefore it accepts all of its options. You can specify a starting page with :page (default is 1). Default :order is "id", override if necessary.
See Faking Cursors in ActiveRecord where Jamis Buck describes this and a more efficient way for MySQL.
# File lib/will_paginate/finder.rb, line 99 def paginated_each(options = {}) options = { :order => 'id', :page => 1 }.merge options options[:page] = options[:page].to_i options[:total_entries] = 0 # skip the individual count queries total = 0 begin collection = paginate(options) with_exclusive_scope(:find => {}) do # using exclusive scope so that the block is yielded in scope-free context total += collection.each { |item| yield item }.size end options[:page] += 1 end until collection.size < collection.per_page total end
Protected instance methods
Does the not-so-trivial job of finding out the total number of entries in the database. It relies on the ActiveRecord count method.
# File lib/will_paginate/finder.rb, line 189 def wp_count(options, args, finder) excludees = [:count, :order, :limit, :offset, :readonly] excludees << :from unless ActiveRecord::Calculations::CALCULATIONS_OPTIONS.include?(:from) # we may be in a model or an association proxy klass = (@owner and @reflection) ? @reflection.klass : self # Use :select from scope if it isn't already present. options[:select] = scope(:find, :select) unless options[:select] if options[:select] and options[:select] =~ /^\s*DISTINCT\b/i # Remove quoting and check for table_name.*-like statement. if options[:select].gsub('`', '') =~ /\w+\.\*/ options[:select] = "DISTINCT #{klass.table_name}.#{klass.primary_key}" end else excludees << :select # only exclude the select param if it doesn't begin with DISTINCT end # count expects (almost) the same options as find count_options = options.except *excludees # merge the hash found in :count # this allows you to specify :select, :order, or anything else just for the count query count_options.update options[:count] if options[:count] # forget about includes if they are irrelevant (Rails 2.1) if count_options[:include] and klass.private_methods.include_method?(:references_eager_loaded_tables?) and !klass.send(:references_eager_loaded_tables?, count_options) count_options.delete :include end # we may have to scope ... counter = Proc.new { count(count_options) } count = if finder.index('find_') == 0 and klass.respond_to?(scoper = finder.sub('find', 'with')) # scope_out adds a 'with_finder' method which acts like with_scope, if it's present # then execute the count with the scoping provided by the with_finder send(scoper, &counter) elsif finder =~ /^find_(all_by|by)_([_a-zA-Z]\w*)$/ # extract conditions from calls like "paginate_by_foo_and_bar" attribute_names = $2.split('_and_') conditions = construct_attributes_from_arguments(attribute_names, args) with_scope(:find => { :conditions => conditions }, &counter) else counter.call end count.respond_to?(:length) ? count.length : count end