OR Impedance Mismatch Object Relational Mapping The LINQ Project Data Access In APIs Today Data Access with DLINQ DLinq For Relational Data Architecture Key Takeaways Querying For Objects When to Use LINQ to SQL?

1. OR Impedance Mismatch

2. OR Impedance Mismatch
•Conflicting type systems
•Conflicting design goals
Database system focuses specifically on the storage and retrieval of data, whereas an object system
focuses specifically on the union of state and behavior for easier programmer manipulation
•Conflicting architectural style
Most database products are built to assume a fundamentally client/server style of interaction, assuming the
database is located elsewhere on the network, and programs accessing the database will be doing so via
some sort of remote access protocol. Object systems assume the precise opposite, and in fact, perform
significantly worse when distributed.
•Differing structural relationships
Relational data stores track entities in terms of relations between tuples and tuplesets; object-oriented
systems instead prefer to track entities in terms of classes, compilation of state and behavior that relates to
one another through IS-A and/or HAS-A style unidirectional connections. Where databases use foreign-key
relationships to indicate relations, objects use references or pointers
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3. OR Impedance Mismatch
•Differing identity constructs
Object systems use an implicit sense of identity to distinguish between objects of similar state (the
ubiquitous this pointer or reference), yet databases require that sense of identity to be explicit via primary
key column or columns. In fact, in modern object-oriented languages an object system cannot be built
without a sense of object identity, whereas relational tables can have no primary key whatsoever, if
desired.
•Transactional boundaries
Object systems do not have any sense of "transactional demarcation" when working with the objects,
whereas database instances must in order to deal with the multi-user requirements of a modern
.
client/server-based system
•Query/access capabilities
Retrieving data stored in a relational database makes use of SQL, a declarative language predicated on
the mathematical theory of relational algebra and predicate. In object systems, the entire object is required
in order to navigate from one object to the next, meaning that the entire graph of objects is necessary in
order to find two disparate parts of data—for a system intended to remain entirely in working memory, this
is of no concern, but for a system whose principal access is intended to be distributed, as relational
database are, this can be a crippling problem.
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4. Object Relational Mapping
Wikipedia defines an ORM as: “a programming technique for
converting data between incompatible type systems in
relational databases and object-oriented programming
languages. This creates, in effect, a "virtual object
database," which can be used from within the programming
language.” An ORM has to provide a facility to map
database tables to domain objects, using a design surface or
wizard. This mapping is in-between your database and
domain model, independent from the source code and the
database. The ORM runtime then converts the commands
issued by the domain model against the mapping into back
end database retrieval and SQL statements. Mapping allows
an application to deal seamlessly with several different
database models, or even databases.
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5. The LINQ Project
C# VB Others…
.NET Language Integrated Query
Standard
DLinq XLinq
Query
(ADO.NET) (System.Xml)
Operators
<book>
<title/>
<author/>
<year/>
<price/>
</book>
Objects SQL WinFS XML

6. Data Access In APIs Today
Sql C onnect i on c = new Queries in
Sql C onnect i on( …) ; quotes
c. Open( ) ;
Sql C m
om and cm = new Sql C m
d om and( Arguments
@ SELEC c. N e, c. Phone
" T am loosely
FR M C
O ust om s cer bound
W ER c. C t y = @
H E i p0"
Results
);
loosely
cm Par am er s. AddW t hVal ue( " @po" ,
d. et i
typed
Compiler
" London" ) ;
at eader dr = c. Execut e( cm ; cannot help
D aR d)
w l e ( dr . R
hi ead( ) ) { catch
st r i ng nam = dr . G St r i ng( 0) mistakes
e et ;

7. Data Access with DLINQ
publ i c cl ass C ust omer Classes
{ describe
publ i c i nt I d; data
Tables are
publ i c st r i ng N e;
am
collections
publ i c st r i ng Phone;
…
}
Query is
Tabl e<C ust om > cust om s =
er er natural part
db. Cust om s;
er of the
language
The
var cont act s = compiler
f r om c i n cust om s
er helps you

8. DLinq For Relational Data
Accessing data with DLinq
Classes
public class Customer { … }
describe data
public class Northwind: DataContext Tables are
{ like collections
public Table<Customer> Customers;
… Strongly typed
Northwind db = new } Northwind(…connection
);
var contacts = Integrated
from c in db.Customersquery syntax
where c.City == "London"
select new { c.Name, c.Phone }; typed
Strongly
results

9. Architecture
f r om c i n db. Cust om s
er
w e c. C t y == " London"
her i Application
sel ect
new { c. N e, c. Phone }
am
LINQ Query Objects SubmitChanges()
Services:
DLinq - Change tracking
- Concurrency control
(ADO.NET) - Object identity
SQL Query Rows SQL or
Stored
Procs
sel ect N e, Phone
am
f r om cust om s
er
w e ci t y = ' London'
her
SQLSer

10. Key Takeaways
Language integrated data access
Maps tables and rows to classes and objects
Builds on ADO.NET and .NET Transactions
Mapping
Encoded in attributes
Relationships map to properties
Manually authored or tool generated
Persistence
Automatic change tracking
Updates through SQL or stored procedures
DataContext
Strongly typed database

11. Querying For Objects

12. Key Takeaways
Language Integrated Query
Compile-time type checking, IntelliSense
SQL-like query syntax
With support for hierarchy and relationships
Intelligent object loading
Deferred or immediate
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13. Updating Objects

14. Key Takeaways
Auto-generated updates
Using optimistic concurrency
Transactions
Integrates with System.Transactions
SQL pass-through
Returning objects from SQL queries
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15. DLinq Summary
Allows access to relational data as objects
Supports Language Integrated Query
Works with existing infrastructure
Unifies programming model for objects,
relational and XML
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16. When to Use LINQ to SQL?
The primary scenario for using LINQ to SQL is when building
applications with a rapid development cycle and a simple
one-to-one object to relational mapping against the Microsoft
SQL Server family of databases. In other words, when
building an application whose object model is structured very
similarly to the existing database structure, or when a
database for the application does not yet exist and there is
no predisposition against creating a database schema that
mirrors the object model
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17. When to Use LINQ to SQL?
I want to… LINQ to SQL is
applicable
Use an ORM solution and my database is 1:1 with my object
model
Use an ORM solution with inheritance hierarchies that are
stored in a single table
Use my own plain CLR classes instead of using generated
classes or deriving from a base class or implementing an
interface
Leverage LINQ as the way I write queries
Use an ORM but I want something that is very performant and
where I can optimize performance through stored procedures
and compiled queries
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