I have a model called "Provision" with a table name "provisions". This model have a attributes with default values in database, because I don't know how to set value through ActiveRecord.
But the great problem is when the default value is null on database and I need to use database function like "GETDATE()" in RoR layer.
I'm wrong in my proporsal?
If your problem has to do with queries, then manipulating the ActiveRecord::Base objects wont be helpful. You can always execute custom SQL with ActiveRecord::Base.connection('SQL STRING').
Edit:
Seems you want to run this on the server: UPDATE provinsions SET provisioned_at = GETDATE() WHERE id = ?. Well that's a piece of cake using ActiveRecord, you can refer to the docs.
# if you don't already have an AR::Base class with the same name (singularized) as your db table create it
class Provision < ActiveRecord::Base
# this is not necessary, it specifies the db table name
# that AR already inferred from the class name
self.table_name = 'provisions'
end
Provision.where(provisioned_at: nil).update_all provisioned_at: Time.zone.now
I have a model in Rails that I would like to use as a basis for another.
For example model: parent will be the template for model child.
I can see two possible options:
(1) Inherit from the first model and then add additional columns
Class Parent < ActiveRecord::Base
Class Child < Parent
(2) Copy the model.rb file and add new features
Class Child < ActiveRecord::Base
In both cases the "Rails" part of the model is created, but what about the database table? I could create the table using create table child as select * from parent where 1=2 and then create migrations to add the additional columns, but it doesn't seem like the "Rails way".
Is there an easy way to create a migration based on an existing table. or am I barking up the wrong tree entirely?
Your (1) is called single table inheritance (STI). Basically you use one table that has both the fields of the parent and the child. You'll also need a column called type to identify the type of the object.
Without more details I can't say if it's a good idea to use STI in your case, but (2) copying model.rb certainly doesn't seem right.
#user = User.new
#user.id returns nil but i need to know it before i save. Is it possible ?
YES you can!
I had the same question and investigated the docs.
The ability to solve this question is very related to your database type in fact.
Oracle and Postgresql do have useful functions to easily solve this.
For MySQL(oracle) or SkySQL(open-source) it seems more complicated (but still possible). I would recommend you avoid using these (MySQL/SkySQL) databases if you need advanced database tools.
First you must try to avoid this situation as much as possible in your application design, as it is dangerous to play with IDs before they get saved.
There may be situation where you don't have any other choice:
For instance when two tables are referencing themselves and for security reason you don't allow DELETE or UPDATE on these tables.
When this is the case, you can use the (PostgreSQL, Oracle) database nextval function to generate the next ID number without actually inserting a new record.
Use it in conjunction with the find_by_sql rails method.
To do this with postgreSQL and Rails for instance, choose one of your rails models and add a class method (not an instance method!).
This is possible with the "self" word at the beginning of the method name.
self tells Ruby that this method is usable only by the class, not by its instance variables (the objects created with 'new').
My Rails model:
class MyToy < ActiveRecord::Base
...
def self.my_next_id_sequence
self.find_by_sql "SELECT nextval('my_toys_id_seq') AS my_next_id"
end
end
When you generate a table with a Rails migration, by default Rails automatically creates a column called id and sets it as the primary key's table. To ensure that you don't get any "duplicate primary key error", Rails automatically creates a sequence inside the database and applies it to the id column. For each new record (row) you insert in your table, the database will calculate by itself what will be the next id for your new record.
Rails names this sequence automatically with the table name append with "_id_seq".
The PostgreSQL nextval function must be applied to this sequence as explained here.
Now about find_by_sql, as explained here, it will create an array containing new objects instances of your class. Each of those objects will contain all the columns the SQL statement generates. Those columns will appear in each new object instance under the form of attributes. Even if those attributes don't exist in your class model !
As you wisely realized, our nextval function will only return a single value.
So find_by_sql will create an array containing a single object instance with a single attribute.
To make it easy to read the value of this very attribute, we will name the resulting SQL column with "my_next_id", so our attribute will have the same name.
So that's it. We can use our new method:
my_resulting_array = MyToy.my_next_id_sequence
my_toy_object = my_resulting_array[0]
my_next_id_value = my_toy_object.my_next_id
And use it to solve our dead lock situation :
my_dog = DogModel.create(:name => 'Dogy', :toy_id => my_next_id_value)
a_dog_toy = MyToy.new(:my_dog_id => my_dog.id)
a_dog_toy.id = my_next_id_value
a_dog_toy.save
Be aware that if you don't use your my_next_id_value this id number will be lost forever. (I mean, it won't be used by any record in the future).
The database doesn't wait on you to use it. If somewhere at any time, your application needs to insert a new record in your my_table_example (maybe at the same time as we are playing with my_next_id_sequence), the database will always assign an id number to this new record immediately following the one you generated with my_next_id_sequence, considering that your my_next_id_value is reserved.
This may lead to situations where the records in your my_table_example don't appear to be sorted by the time they were created.
No, you can't get the ID before saving. The ID number comes from the database but the database won't assign the ID until you call save. All this is assuming that you're using ActiveRecord of course.
I had a similar situation. I called the sequence using find_by_sql on my model which returns the model array. I got the id from the first object of the arry. something like below.
Class User < ActiveRecord::Base
set_primary_key 'user_id'
alias user_id= id=
def self.get_sequence_id
self.find_by_sql "select TEST_USER_ID_SEQ.nextval as contact_id from dual"
end
end
and on the class on which you reference the user model,
#users = User.get_sequence_id
user = users[0]
Normally the ID is filled from a database sequence automatically.
In rails you can use the after_create event, which gives you access to the object just after it has been saved (and thus it has the ID). This would cover most cases.
When using Oracle i had the case where I wanted to create the ID ourselves (and not use a sequence), and in this post i provide the details how i did that. In short the code:
# a small patch as proposed by the author of OracleEnhancedAdapter: http://blog.rayapps.com/2008/05/13/activerecord-oracle-enhanced-adapter/#comment-240
# if a ActiveRecord model has a sequence with name "autogenerated", the id will not be filled in from any sequence
ActiveRecord::ConnectionAdapters::OracleEnhancedAdapter.class_eval do
alias_method :orig_next_sequence_value, :next_sequence_value
def next_sequence_value(sequence_name)
if sequence_name == 'autogenerated'
# we assume id must have gotten a good value before insert!
id
else
orig_next_sequence_value(sequence_name)
end
end
end
while this solution is specific to Oracle-enhanced, i am assuming the other databases will have a similar method that you could redefine.
So, while it is definitely not advised and you want to be absolutely sure why you would not want to use an id generated by a sequence, if it is needed it is most definitely possible.
It is why I love ruby and Ruby on Rails! :)
In Oracle you can get your current sequence value with this query:
SELECT last_number FROM user_sequences where sequence_name='your_sequence_name';
So in your model class, you can put something like this:
class MyModel < ActiveRecord::Base
self.sequence_name = 'your_sequence_name'
def self.my_next_id_sequence
get_data = self.find_by_sql "SELECT last_number FROM user_sequences where sequence_name='your_sequence_name'"
get_data[0].last_number
end
end
And finally, in controller you can get this value with this:
my_sequence_number = MyModel.my_next_id_sequence
So, there is no need to get your next value by using NEXTVAL and you won't lose you ID.
What you could do is User.max(id). which will return the highest ID in the database, you could then add 1. This is not reliable, although might meet your needs.
Since Rails 5 you can simply call next_sequence_value
Note: For Oracle when self.sequence_name is set, requesting next sequence value creates side effect by incrementing sequence value
I have to model an association structure and the association is divided into divisions/subdivisions/sections etc. So I've created a simple Entity-Attribute Model:
I'd like to use rail's single-table-inheritance but it seems like this works only if the type column is a string. My question is how to achieve this with my approach? Since I'm using a foreign key as "type" I'd have to query the "type name" first. Has anybody done this before?
I would recommend adding a String "type" attribute to your structure table to satisfy single table inheritance, and to add before_save callbacks to set correct values on either table.
Say you have a StructureType with name "Basic". In Rails that means you'd want to have class hierarchy:
Structure < ActiveRecord::Base
BasicStructure < Structure
In Structure class add:
before_create :set_structure_type_fk
def set_structure_type_fk
self.structure_type = StructureType.find_by_name(\
self.class.name.gsub(/Structure/, '').downcase)
end
Hope this helps.
Note that this approach means that StructureType.name should be immutable: once created it should never be changed, except by a database migration that updates both tables correspondingly.
K
Hi so I have a list of 'areas' that are just areas of my site. I want to be able to insert them in to the database and say what type of area they are, this could be the name of the model that inherits from a base Area. This way I can write a generic controller which uses different views and different logic from the loaded model to decide how the page should act.
Trouble is I have no idea how to do this as I'm new to rails.. Any pointers? (or anyone saying "don't do it like that! do it like this!" would also be much appreciated)
Thanks
I have just found Single Table Inheritance. This appears like it will solve my problem
for more information read here:
http://juixe.com/techknow/index.php/2006/06/03/rails-single-table-inheritance/
You'll want to create a column named :type
Rails will automatically use the type column to determine the class of the child class.
Then you can do something like:
class Area < ActiveRecord::Base
end
class UserPage < Area
end
So then when you do
UserPage.create( :key => 'value')
It will create an entry in your areas table with the type field set to UserPage.