ActiveRecord and use of has_one & has_many
Consider this simple model, where a Project
has one ProjectType
and, naturally many Projects
can be of that type.
So a Project
has_one :project_type
(called type
) and a ProjectType
has_many :projects
.
In my migration I put (simplified for this example)
create_table :projects do |t|
t.string :name, :null => false
t.integer :type
end
create_table :project_types do |t|
t.string :name, :null => false
end
My Project class looks like this (again simplified for this example)
#!usr/bin/ruby
require 'active_record'
class Project < ActiveRecord::Base
has_one :type, :class_name => 'ProjectType'
end
And my ProjectType looks like
#!usr/bin/ruby
require 'active_record开发者_运维百科'
class ProjectType < ActiveRecord::Base
has_many :projects
end
I've written a simple Unit Test to check this works
#test creation of Projects and related objects.
def test_projects_and_etc
pt = ProjectType.create(:name => 'Test PT')
project = Project.create(:name => 'Test Project', :type => pt)
assert project.type.name == 'Test PT', "Wrong Project Type Name, expected 'Test PT' but got '#{project.type.name}'."
# clean up
project.destroy
pt.destroy
end
This test throws an error at the assert, saying
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: project_types.project_id: SELECT "project_types".* FROM "project_types" WHERE ("project_types".project_id = 1) LIMIT 1
The SQL seems to be assuming that there is a project_id
field in the project_types
table but that makes no sense if a ProjectType
can be associated with many Projects
. I suspect that my problem is something to do with my wanting to be able to refer to the ProjectType
as project.type
not project.project_type
, but I am not sure how I'd fix this.
You need to use belongs_to instead of has_one on the project model.
You also need to add a project_type_id column on to the projects table.
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