Invalid source reflection macro?

I'm trying to model a somewhat complicated genetic relationship in

class Gene < ActiveRecord::Base
  belongs_to :species, :foreign_key => "species_id", :class_name =>
  has_many :genes_orthogroups, :class_name => 'GenesOrthogroups' #
join table model
  has_many :orthogroups, :through => :genes_orthogroups
  has_many :orthologs, :through => :orthogroups, :source
=> :genes, :conditions => 'gene.species_id != ortholog.species_id'
class GenesOrthogroups < ActiveRecord::Base
  belongs_to :gene
  belongs_to :orthogroup
class Orthogroup < ActiveRecord::Base
  has_many :genes_orthogroups, :class_name => 'GenesOrthogroups'
  has_many :genes, :through => :genes_orthogroups
  belongs_to :species_pair, :class_name => 'SpeciesPair', :foreign_key
=> "species_pair_id"
class SpeciesPair < ActiveRecord::Base
  belongs_to :species1, :foreign_key => "species1_id", :class_name =>
  belongs_to :species2, :foreign_key => "species2_id", :class_name =>
  has_many :orthogroups
class Species < ActiveRecord::Base
  has_many :genes
  has_many :species_pairs, :class_name => 'SpeciesPair'

I'm trying to print out a list of the orthologs with a partial. The
list is generated with:
  def find_ortholog_list(gene_id)
    ortholog_list = []
    Gene.find(gene_id).orthologs.each do |ortholog|
      ortholog_list << ortholog

I get the following error message:
Invalid source reflection macro :has_many :through for
has_many :orthologs, :through => :orthogroups. Use :source to specify
the source reflection.

If I change :source to :gene instead of :genes, I get:
Could not find the source association(s) :gene in model Orthogroup.
Try 'has_many :orthologs, :through => :orthogroups, :source =>
<name>'. Is it one of :genes_orthogroups, :genes, or :species_pair?

Essentially, any given gene should have zero, one, or multiple
orthologs in another species. These are grouped into orthogroups, each
consisting of genes from two species, and representing a many-to-many
relationship. Genes in an orthogroup which are of the same species
would be paralogs instead. Because there are more than just two
species, a gene may participate in multiple orthogroups (up to one for
each additional species). The generalization for ortholog/paralog is
"homolog" or "homologue," incidentally.

I've based most of this code on the p.369 explanation "Using Models as
Join Tables" in Agile Web Development with Rails (3rd Ed). I'm not
really sure why it fails, except for the fact that I'm trying to use
Gene as its own source for defining orthologs. What's the proper way
to define this relationship?

Apologies for the complexity of the model. I couldn't think of any
layman's examples.

Thanks so much,

The :orthologs association is the problem - Rails doesn't support
nesting :through associations. I recall there being a plugin around
someplace to do it, so you may want to look into that.

Depending on what you need, a simple instance method may work as well.
For example (on Gene):

def orthologs
  genes_orthogroups.ortholog_groups_for(self).map { |g| g.gene }

On GenesOrthogroup:
named_scope ortholog_groups_for { |g| { :include => :gene, :conditions
=> ['genes.species_id != ?', g.species_id] } }

(not tested, but should be close to working)

A couple general things:

- model names should be singular (GenesOrthogroup rather than
GenesOrthogroups). Otherwise you'll eventually run into issues.

- when writing SQL fragments in conditions, table names are plural (so
genes.whatever rather than gene.whatever).

- the association macros have sensible defaults, so you can leave some
options out. For instance, the :species association in Gene can be
simplified to 'belongs_to :species' - Rails will find the correct FK
(species_id) and class (Species).

--Matt Jones

The plural model name comes from the Agile Development book. They have
an example--categories_products, linking categories to products--which
they turn into a model. Should it be renamed category_product? Or is
categories_product correct? I'm a bit confused about this, so thanks
for bringing it up.

I'll look around for the plugin. Thanks so much, Matt!


I'll look at the rest of your example later, but...

John Woods wrote:

The plural model name comes from the Agile Development book. They have
an example--categories_products, linking categories to products--which
they turn into a model. Should it be renamed category_product?


If I had to choose one of those options, I'd probably use
CategoryProduct and rename the table. However, I think neither option
is very good. What most people here seem to recommend is this: when
your join table becomes a model, take the time to come up with an
appropriate name for it.

For example, in this case, we might go from Category habtm Products
(with categories_products table) to

class Category
  has_many :categorizations
  has_many :products, :through => :categorizations

(likewise for Product)

class Categorization
  belongs_to :category
  belongs_to :product

Choosing an appropriate name for the join model will make future
development much easier and clearer.


So the plugin appears to work. FYI, it's here:

Unfortunately, I'm still having trouble setting up the condition. I've
renamed a few things and added appropriate associations:

class Gene < ActiveRecord::Base
  belongs_to :species
  has_many :gene_orthogroup_linkers, :class_name =>
  has_many :orthogroups, :through => :gene_orthogroup_linkers
  has_many :ortholog_orthogroup_linkers, :class_name =>
'GeneOrthogroupLinker', :through => :orthogroups, :source
=> :gene_orthogroup_linkers
  has_many :orthologs, :source => :gene, :through
=> :ortholog_orthogroup_linkers, :conditions => 'genes.species_id !=

GeneOrthogroupLinker is the class that used to be GenesOrthogroups.

Mysql::Error: Unknown column 'genes_2.species_id' in 'where clause':
SELECT `genes`.* FROM `genes` INNER JOIN gene_orthogroup_linkers ON
( = gene_orthogroup_linkers.gene_id ) INNER JOIN orthogroups
ON ( gene_orthogroup_linkers.orthogroup_id = ) INNER
JOIN gene_orthogroup_linkers gene_orthogroup_linkers_2 ON
( = gene_orthogroup_linkers_2.orthogroup_id )
WHERE (gene_orthogroup_linkers_2.gene_id = 556 AND genes.species_id !
= genes_2.species_id)

So, it looks to me like the problem is that it selects from genes but
has no inner join to genes_2, only to gene_orthogroup_linkers_2. How
do I get it to INNER JOIN genes genes_2 ON
( gene_orthogroup_linkers_2.gene_id = AND genes.species_id !
= genes_2.species_id)?

It seems like as a work-around, I could add a species_id column on the
GeneOrthogroupLinker model, but that doesn't seem like the cleanest

Incidentally, it seems unnecessary to join with orthogroups (why not
just join the two linkers with orthogroup_id?). Is this easily
fixable, or better left as-is?

I also tried the named_scope, but it doesn't seem right for this


Have you tried passing the missing join directly, via the :join

--Matt Jones