My development environment:

• Ubuntu 9
• Ruby 1.9.1/1.8.7 (rvm)
• Rails 2.3.5
• Mysql 5.0
• Apache Passenger

Below is the part of the program flow to represent the issue.

Request comes:

#action
def create
  begin
    @report = Report.new(params[:report])
    ...  
  rescue LocationNotFound => e
    ...
  end
end

Report constructor:

class Report
  attr_accessor :locations

  def initialize(params = {})
    @locations = params[:locations] ? fetch_locations(params[:locations]) : []
  end
  ...
end

fetch_locations:

def fetch_locations(loc_names)
  Rails.logger.debug "LOC_NAMES: " + loc_names.inspect
  ls = Location.find(:all, :conditions => [ # line 57
    "locations.name in (#{loc_names.map{'?'}.join(',')})",
    *loc_names
  ], :include => [:sample_summaries, :samples]) # loc_names will never be empty
  ...
end

Location model:

class Location < ActiveRecord::Base
  has_many :sample_summaries
  has_many :samples, :through => :sample_summaries
  ...
end

Now, the first time (after passenger restart) this runs fine and does the job. Most of the consequent times I get the error:

Mar-11 11:01:00 #15098 DEBUG: LOC_NAMES: ["Moscow, RF", "London, UK"]
Mar-11 11:01:00 #15098 DEBUG:   Location Load (0.0ms)   SELECT * FROM `locations` WHERE (locations.name in ('Moscow, RF','London, UK'))
Mar-11 11:01:00 #15098 DEBUG:   SampleSummary Load (0.0ms)   SELECT `sample_summaries`.* FROM `sample_summaries` WHERE (`sample_summaries`.location_id IN (1,3))
Mar-11 11:01:00 #15098 DEBUG:   SampleSummary Columns (0.0ms)   SHOW FIELDS FROM `sample_summaries`
Mar-11 11:01:00 #15098 FATAL:
NoMethodError (You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.include?):
  app/models/report.rb:57:in `fetch_locations'
  app/models/report.rb:9:in `initialize'
  app/controllers/report_controller.rb:11:in `new'
  app/controllers/report_controller.rb:11:in `create'

Looks quite random to me. Any ideas?

P.S. I also tried to wrap the query in uncached block, but that didn't change anything.

EDIT

Here is what SampleSummary model looks like:

class SampleSummary < ActiveRecord::Base
  has_many :samples
  belongs_to :location
  ... #validations

  default_scope :include => :samples, :order => 'rss_ts desc'
  ...
end