Implementing the DataMapper Pattern With .Net 1.1

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Implementing the DataMapper Pattern With .Net 1.1
Code Camp II, Waltham, MA October 16th, 2004

• Don Demsak
• www.donxml.com

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Data Mapper Pattern Definition
A layer of Mappers that moves data between
objects and a database while keeping them
independent of each other and the mapper itself.
(Patterns of Enterprise Application Architecture
page 165 by Martin Fowler)
Mapper (def.) An object that sets up a
communication between two independent objects.
(Patterns of Enterprise Application Architecture
page 473 by Martin Fowler)
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Alternates To Data Mappers

• System.Data.DataSet
• No Domain Objects
• Strongly Typed DataSet
• Domain Objects in terms of data only
• Table Module
• A single instance that handles the business logic
  for all rows in a database table or view.
• Transaction Scripts
• Organizes business logic by procedures where each
  procedure handles a single request from the
  presentation.

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Layered Architecture
Which Transport Object?
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Data Transport Choices

• DataReader
• Fast But Only 2 Dimensional
• DataSet
• Easy But Still Only 2 Dimensional
• XmlReader
• Multi- Dimensional
• XmlSerialization
• XPathNavigator
• Mostly Unknown

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The Example (Using Northwind)

• Get a list of all employees including
• Each employees territories
• Each territories region

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Northwind ERD
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Base Line Of Business Diagram
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GetEmployee SQL
SELECT Employee.EmployeeId as "Employee.Id", Em
ployee.LastName as "Employee.LastName", Employee
.FirstName as "Employee.FirstName", Employee.Tit
le as "Employee.Title", Employee.BirthDate as
"Employee.BirthDate", Territory.TerritoryId as
"Territory.Id", RTRIM(Territory.TerritoryDescrip
tion) as "Territory.Description", Region.RegionI
D as "Region.Id", RTRIM(Region.RegionDescription
) as "Region.Description" from dbo.Employees
Employee INNER JOIN dbo.EmployeeTerritories
et on Employee.EmployeeID et.EmployeeID INNER
JOIN dbo.Territories Territory on
et.TerritoryID Territory.TerritoryID INNER
Join dbo.Region Region on Region.RegionID
Territory.RegionID order by Employee.EmployeeID,
Territory.TerritoryId, Region.RegionId
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Base Data Access Layer Diagram
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Code Demo

• Which Transport Object?

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Performance Results

• DataReader 341
• DataSet 411
• XmlReader (XmlSerialization) 542
• XPathNavigator - 450

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DataReader Results

• Pros
• Fast
• Cons
• Domain Objects Need To Know the Underlying DB
  implementation
• Connection Controlled by Application Layer
• Domain Objects Need to Know Too Much About its
  children
• Need to Create Internal Setters For Some
  Properties

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DataSet Results

• Pros
• Easy to Learn
• Cons
• Domain Objects Need to Know Too Much About its
  children
• Need to Create Internal Setters For Some
  Properties

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XmlSerialization Results

• Pros
• Very Little Mapping Code Required
• Cons
• Connection Controlled by Application Layer
• Domain Constructors Must Be Public
• Collection Add Methods Must Be Public

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XPathNavigator Resuls

• Pros
• Complies with Domain Driven Design Principles
• DataStore Agnostic
• Declarative In Coding Style
• Cons
• Learning Curve (XPath)
• Slower than DataReader

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Questions?
Contact Info Web Site www.donxml.com Email
don_at_donxml.com