For Eg.
I have a table with 4 Columns
Id
P Id
Name
Gender
And a Model Named Employee with same fields
Now If I make any change in table structure
For Eg. Added Birth date Column
Is there any way, the model will be updated automatically by reading the table?
Or is there any way that i defined the table name, the columns will be fetched in Model Itself.
You should map your model with your table using ORMs like entity framework. In your demand, if you use a database-first approach, then whenever you change your table, you can update your model by the wizard. For more information click here
I come from a Java background and I have started learning Ruby on Rails. Consider the following code mentioned in http://guides.rubyonrails.org/active_record_basics.html
class Product < ActiveRecord::Base
end
The guide mentions that this creates a model Product mapped to a table products (using the pluralized mechanism of ruby). It also mentions, 'By doing this you'll also have the ability to map the columns of each row in that table with the attributes of the instances of your model.'
But we did not declare any attributes inside the model Product. How does it know what are its attributes?
One assumption: Every attribute of table is made as an attribute of model. Is it true? Then, do we create the SQL table first? If I change the table later on (adding new columns, say) does it also change my model dynamically?
The important distinction is that we're talking about ActiveRecord models, i. e. subclasses (direct and indirect) of ActiveRecord::Base, those that use its persistence mechanism. The following is not true for Rails models in general. But then again, for non-AR models the question makes no sense :)
Every attribute of table is made as an attribute of model. Is it true?
Yes.
Then, do we create the SQL table first?
Exactly. rails g model creates a model file and a migration that contains a declaration for a table behind the model. So before using your model, you have to run the migration first.
If I change the table later on (adding new columns, say) does it also change my model dynamically?
Now that's tricky. It's most certainly true if the application is reloaded after the changes (e. g. in development mode this happens every now and then), since the model class will be reconstructed. So the answer is yes, most of the time.
This is, however, only about the internal structures of the model class (visible in e. g. Model.columns) you don't always have to care about. When fetching data, all the columns of the result set will be mapped to attributes of the model objects. So this keeps even custom columns you specify in SELECTs:
Thing.select(:id, "1 AS one").first.attributes
#> SELECT "things"."id", 1 AS one FROM "things" ORDER BY "things"."id" ASC LIMIT 1
# => {"id"=>1, "one"=>1}
It works like this:
class Product < ActiveRecord::Base
Product is subclassed to ActiveRecord::Base (you know subclassing from Java, right?).
ActiveRecord::Base can be seen here:
Active Record objects don't specify their attributes directly, but rather infer them from
# the table definition with which they're linked. Adding, removing, and changing attributes
# and their type is done directly in the database. Any change is instantly reflected in the
# Active Record objects. The mapping that binds a given Active Record class to a certain
# database table will happen automatically in most common cases, but can be overwritten for the uncommon ones.
You can read through the other code; in short, it means that ActiveRecord uses the SQL schema to populate the respective attributes.
--
Because your model is a Class, ActiveRecord will basically create a series of setter/getter instance methods with the values from your db.
When you invoke said class, ActiveRecord::Base will populate the respective instance methods with the values in your db, allowing you to call #product.name etc.
Models in rails are just an easy way of binding the data from the database. Model is the data.
A model represents a table and will have all the columns of that table as its attributes.
The model in point is Product. By rails conventions, this model is directly mapped to products table in the database and will have all the attributes that the table has as its columns.
Models and tables are interlinked and models serve as an easy abstract layer over the actual data to provide ease of work and additional validations and stuff like that.
You only have to declare specific attributes in a migration (which creates the tables). Otherwise, ActiveRecord makes a some key assumptions:
name of the table = lowercase version of class name = products
primary key = id
Then it can use raw SQL when it starts the connection to get a list of attributes from the table:
DESCRIBE table products;
This gives it a full listing of the fields in the table. It sets up attributes in each instance of the class based on these fields.
Right. So I am new to MVC.
I have two tables. tblCustomer_Details and tblCustomer_Location. There is a relationship between the two tables by CustomerID.
Now, how do I display few columns from one table and few from others?
I have tried using View Model. I created a new model named ViewModel and added Lists of both tables.
How do I proceed from here?
suppose I have a legacy database that I can't refactor in any way.
I have a database table where a record contains fields that should belong to two logical different objects whose relation is actually 1:1, now to have my code cleaner I would like to split this single object in two in my application model.
So suppose now I have a table with n fields where the first x fields should be mapped by a model in my rails app, and the remaining n - x fields should be mapped in another model, is there a simple way to achieve this in active records?
Thanks,
Gnagno
I want to show list of order details including count of ordered material and name of customer.
There are three tables order_details, order_item_details and customer_details
for this scenario how to create view model and linq
my db structure
customer_details === customer_id,customer_name
|||||
order_details === order_id, order_number, customer_id
|||||
order_item_details === order_item_id, order_id , item_id
and i want to show------
order_details.order_number, customer_details.customer_name, count(order_item_details.order_item_id)
Create a view in your database that links the three tables appropriately, then point your ORM (EF?) at the view, and you will get a read-only view of your table with appropriate models. If what you're doing is particularly complex, you can create a stored procedure and use the same technique.
If you are in fact using Entity Framework, here's an SO answer that tells you how to create custom entities in EF 4 (first answer by Kevin Castle):
Entity framework 3.5, mapping stored procedure results to custom entity