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<title>Database Objects</title>
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<h1>
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Quick Start<!-- InstanceEndEditable --></h1>
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<p>Implementing a database table using classes and the DatabaseObjects library
involves creating two classes. The first class, represents a database table,
while the second class represent each database record in the table. For the
classes to "hook into" the library the first class (or collection
class) must implement the <strong>IDatabaseObjects</strong> interface while
the second class must implement the <strong>IDatabaseObject</strong> interface.
By implementing each of the <strong>IDatabaseObjects</strong>' and <strong>IDatabaseObject</strong> functions
the collection class can specify which table it is tied to, the table's unique
field, whether it should only represent a subset of the table, how the table
should be sorted, if there are any related tables, etc. while the second class
can specify how a database record is to be copied to and from the class. With
the interfaces implemented the DatabaseObjects library can then automatically
generate the appropriate SQL statements for common database functions such
as inserting a new record, updating an existing record, searching through a
table, enumerating through a set of records, returning the number of records
in a table, etc.</p>
<p>The diagram below depicts how a <strong>Products</strong> database table might
be implemented using the library. Two classes would be required; a <strong>Products </strong>class
that implements the <strong>IDatabaseObjects</strong> interface and a<strong> Product</strong> class
that implements the <strong>IDatabaseObject</strong> interface. Once the interfaces
have been implemented the library can then be used with the DatabaseObjects
library's set of predefined, generic functions to automatically generate and
execute the necessary SQL statements. For example, the <strong>Count </strong>property
in the <strong>Products</strong> class could call one of the predefined DatabaseObjects
functions: <strong>ObjectsCount</strong>. This function creates an SQL statement
using the value returned from <strong>IDatabaseObjects_TableName</strong> (in
this case "Products") to generate the following: <code>SELECT COUNT(*)
FROM Products</code>. The SQL statement is then executed and the result returned.
If the <strong>DBO.ObjectsCount</strong> function was called by passing a <strong>Customers</strong> class
which had implemented <strong> IDatabaseObjects_TableName</strong> to return "Customers" then
the <strong>DBO.ObjectsCount</strong> function would generate and execute the
statement:<code> SELECT COUNT(*) FROM Customers</code>. This basic technique
is used by the DatabaseObjects library and it's set of generic functions.</p>
<p> </p>
<p><img src="images/databaseobjects_classdiagram.jpg" width="479" height="764"></p>
<h3><strong>Code Example</strong></h3>
<p>This example demonstrates using the DatabaseObjects library with the <strong>Products</strong> table
from Microsoft's Northwind database. The Microsoft Access version of the Northwind
database is included with Visual Basic 6 and by default is located at: <em>C:\Program
Files\Microsoft Visual Studio\VB98\nwind.mdb. </em>The following example assumes
that the database exists at this location - although this can be modified.
The database can also be downloaded <a href="http://www.microsoft.com/downloads/details.aspx?FamilyID=C6661372-8DBE-422B-8676-C632D66C529C&displaylang=EN">here</a> from
the Microsoft website. A MySQL version of the Northwind database is also bundled
with the demonstration
program.</p>
<ol>
<li>Run <strong>Visual Basic 6</strong></li>
<li>Create a new <strong>ActiveX DLL</strong></li>
<li>Rename <strong>Project1</strong> to <strong>NorthwindDB</strong></li>
<li>Rename <strong>Class1</strong> to <strong>Products</strong></li>
<li>Add a new class, name it <strong>Product</strong></li>
<li>Add the DBO library project
<ol>
<li>Select <strong>File > Add Project</strong></li>
<li>Select the <strong>Existing</strong> tab</li>
<li>Navigate to the <strong>DBO.vbp</strong> project</li>
<li>Select <strong>Open</strong></li>
</ol>
</li>
<li>Select the <strong>NorthwindDB</strong> project</li>
<li>Select <strong>Project > References</strong></li>
<li>Select the <strong>DBO</strong> library and the<strong> Microsoft ActiveX
Data Objects 2.8 Library </strong>references</li>
</ol>
<h3><strong><br>
Product Class</strong></h3>
<ol>
<li>Open the <strong>Product</strong> class</li>
<li>At the top of the class type <strong>Implements IDatabaseObject</strong></li>
<li>Select the <strong>IDatabaseObject</strong> from the object list (combo box at the top left)</li>
<li>Select each function (combo box at the top right) to automatically
generate the <strong>IDatabaseObject </strong>functions. All of the functions
must be implemented even if they are to be left blank.</li>
<li>Fill in the remainder of the class so that it looks like the following:</li>
</ol>
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'Product class</span>
<span class="keyword">Implements</span> <span class="keyword"><a href="IDatabaseObject.htm">IDatabaseObject</a>
Public </span>Name <span class="keyword">As String<br>Public </span>UnitPrice<span class="keyword"> As Currency<br></span>
<span class="keyword">Private</span> plngProductID<span class="keyword"> As Long</span>
<span class="comment">
'Implement the IDatabaseObject functions</span>
<span class="keyword">Private Property Let</span> <a href="IDatabaseObject_DistinctValue.htm">IDatabaseObject_DistinctValue</a>(<span class="keyword">ByVal</span> RHS <span class="keyword">As Variant</span>)
<span class="comment">
'This function is called when the Product is loaded or
'this is a new object which has not yet been saved and is identified
'by an identity or autoincrement field (<a href="IDatabaseObjects_DistinctFieldAutoIncrements.htm">IDatabaseObjects_DistinctFieldIsAnIdentityField</a>)
'Store the Product ID
</span> plngProductID = RHS
<span class="keyword">End Property</span>
<span class="keyword">Private Property Get</span> <a href="IDatabaseObject_DistinctValue.htm">IDatabaseObject_DistinctValue</a>() <span class="keyword">As Variant</span>
<span class="comment">
</span><span class="comment"> 'Return the distinct value for the Product which in this case is the ProductID (plngProductID)</span>
IDatabaseObject_DistinctValue = plngProductID
<span class="keyword">End Property</span>
<span class="keyword">Private Property Get</span> <a href="IDatabaseObject_IsSaved.htm">IDatabaseObject_IsSaved</a>() <span class="keyword">As Boolean</span>
<span class="comment"> 'Return whether the object has been saved to the database
'If the object is new then plngProductID will be 0
'This property is essentially used to determine whether to
'perform either an INSERT or UPDATE SQL command
</span>
IDatabaseObject_IsSaved = plngProductID <> 0
<span class="keyword">End Property</span>
<span class="keyword">Private Property Let</span> <a href="IDatabaseObject_IsSaved.htm">IDatabaseObject_IsSaved</a>(<span class="keyword">ByVal</span> RHS <span class="keyword">As Boolean</span>)
<span class="keyword">End Property</span>
<span class="keyword">Private Sub</span> <a href="IDatabaseObject_Load.htm">IDatabaseObject_Load</a>(<span class="keyword">ByVal</span> objFields <span class="keyword">As</span> DBO.SQLFieldValues)
<span class="comment"> 'objFields will be populated with all of the fields from 1 record of the Products' table
'Copy the fields from the database (via objFields) and store them in the appropriate variables
</span> Me.Name = objFields("ProductName")<br> Me.UnitPrice = objFields("UnitPrice")<br>
<span class="keyword">End Sub</span>
<span class="keyword">Private Function </span><a href="IDatabaseObject_Save.htm">IDatabaseObject_Save</a>() <span class="keyword">As</span> DBO.SQLFieldValues
<span class="keyword">Dim</span> objFields <span class="keyword">As</span> SQLFieldValues<br> <span class="keyword">Set</span> objFields = <span class="keyword">New</span> SQLFieldValues
objFields.Add "ProductName", Me.Name<br> objFields.Add "UnitPrice", Me.UnitPrice
<span class="keyword">Set</span> IDatabaseObject_Save = objFields
<span class="keyword">End Function</span></pre>
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<h3><br>
Products Class</h3>
<ol><li>Open the <strong>Products</strong> class</li>
<li>At the top of the class type <strong>Implements IDatabaseObjects</strong></li>
<li>Just as before, select the <strong>IDatabaseObjects</strong> from the object
list (combo box at the top left)</li>
<li>Select each function (combo box at the top right) to automatically generate
the <strong>IDatabaseObjects </strong>functions. All of the functions must
be implemented.</li>
<li>Fill in the remainder of the class so that it looks like the following:</li>
</ol>
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<td><pre><span class="comment">
'Products class</span>
<span class="keyword">Implements</span> <a href="IDatabaseObjects.htm">IDatabaseObjects</a><br>
<span class="keyword">Public Property Get</span> Enumerator() <span class="keyword">As</span> IUnknown
<span class="comment"> 'This property will allow a For Each enumeration to be used</span>
<span class="comment"> 'colProducts must be declared static otherwise it will be released
'at the end of the function call and the enumerator will become invalid
</span>
<span class="keyword"> Static</span> colProducts <span class="keyword">As</span> Collection
<span class="keyword">Set</span> colProducts = dbo.<a href="DatabaseObjects_ObjectsCollection.htm">ObjectsCollection</a>(Me)<br>
<span class="keyword">Set</span> Enumerator = colProducts.[_NewEnum]
<span class="keyword">End Property</span><br> <br>
<span class="comment">'Implement the IDatabaseObjects functions</span><span class="keyword">
Private Function</span> <a href="IDatabaseObjects_DistinctFieldAutoIncrements.htm">IDatabaseObjects_DistinctFieldAutoIncrements</a>() <span class="keyword">As Boolean</span>
<span class="comment">'The ProductID field is an automatically incrementing field</span>
IDatabaseObjects_DistinctFieldAutoIncrements = <span class="keyword">True</span>
<span class="keyword">End Function
Private Function</span> <a href="IDatabaseObjects_DistinctFieldName.htm">IDatabaseObjects_DistinctFieldName</a>() <span class="keyword">As String</span>
<span class="comment"> 'The ProductID field uniquely identifies each product record in the table</span>
IDatabaseObjects_DistinctFieldName = "ProductID"
<span class="keyword">End Function
Private Function </span><a href="IDatabaseObjects_ItemInstance.htm">IDatabaseObjects_ItemInstance</a>() <span class="keyword">As</span> dbo.IDatabaseObject
<span class="comment">'Return a new instance of the class that is associated with this collection</span>
<span class="keyword">Set</span> IDatabaseObjects_ItemInstance = <span class="keyword">New</span> Product
<span class="keyword">End Function
Private Function </span><a href="IDatabaseObjects_KeyFieldName.htm">IDatabaseObjects_KeyFieldName</a>() <span class="keyword">As String</span>
<span class="comment">
'The ProductName field is a unique field within the product table
'and is used by the DBO.ObjectByKey function</span>
IDatabaseObjects_KeyFieldName = "ProductName"
<span class="keyword">End Function
Private Function</span> <a href="IDatabaseObjects_OrderBy.htm">IDatabaseObjects_OrderBy</a>() <span class="keyword">As</span> DBO.SQLSelectOrderByFields
<span class="comment"> 'When enumerating through the collection (using ObjectByOrdinal or ObjectsCollection)
'then the Product objects should be ordered by ProductName</span>
<span class="comment"> 'If this function returns Nothing then the records are not sorted</span>
<span class="keyword">Set</span> IDatabaseObjects_OrderBy = <span class="keyword">New</span> SQLSelectOrderByFields
IDatabaseObjects_OrderBy.Add "ProductName", dboOrderAscending
<span class="keyword">End Function
Private Function </span><a href="IDatabaseObjects_TableName.htm">IDatabaseObjects_TableName</a>() <span class="keyword">As String</span>
<span class="comment">'Return the database table that this collection uses</span>
IDatabaseObjects_TableName = "Products"
<span class="keyword">End Function</span>
<span class="keyword">
Private Function</span> <a href="IDatabaseObjects_Subset.htm">IDatabaseObjects_Subset</a>() <span class="keyword">As</span> dbo.SQLConditions
<span class="comment"> 'Leave this function blank to include all of the product records</span>
<span class="keyword">End Function
Private Function</span> <a href="IDatabaseObjects_TableJoins.htm">IDatabaseObjects_TableJoins</a>(<span class="keyword">ByVal</span> objPrimaryTable <span class="keyword">As</span> SQLSelectTable, <span class="keyword">ByVal</span> objTables <span class="keyword">As</span> dbo.SQLSelectTables) <span class="keyword">As</span> dbo.SQLSelectTableJoins
<span class="comment"> 'Leave this function blank to not join to any tables </span>
<span class="keyword">End Function</span></pre></td>
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<ol>
<li>Enable the<strong> For Each</strong> enumeration
<ol>
<li> Select Tools > Procedure Attributes</li>
<li> Select <strong>Enumerator</strong> in the Name list</li>
<li>Click Advanced</li>
<li>Enter -4 for the 'Procedure ID'</li>
<li>This will allow the <strong>For Each</strong> command to be used on
the <strong>Products</strong> collection</li>
</ol>
</li>
</ol>
<h3>NorthwindDatabase Class</h3>
<ol>
<li>Add a new class to the <strong>NorthwindDB</strong> project, name it <strong>NorthWindDatabase</strong></li>
<li>Set the instancing property for the <strong>NorthWindDatabase</strong> class
to <strong>6-GlobalMultiUse</strong></li>
<li>Add the following code to the <strong>NorthWindDatabase</strong> class</li>
<li>This will connect to the nwind.mdb database. The nwind.mdb database is
available <a href="http://www.microsoft.com/downloads/details.aspx?FamilyID=C6661372-8DBE-422B-8676-C632D66C529C&displaylang=EN" target="_blank">here</a> at
the Microsoft website.</li>
</ol>
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<span class="keyword">Public Sub</span> Connect()
<span class="keyword">Const</span> cstrDatabaseFilePath <span class="keyword">As String</span> = "C:\Program Files\Microsoft Visual Studio\VB98\nwind.mdb"
Dim strConnection As String
<span class="keyword"> </span> strConnection = _<br> "Data Source=" & cstrDatabaseFilePath & ";" & _<br> "Provider=Microsoft.Jet.OLEDB.4.0;Jet OLEDB:Database Password=;"
DBO.Connect strConnection, dboConnectionTypeMicrosoftAccess
<span class="keyword">End Sub</span>
<span class="keyword">Public Property Get </span>Products() <span class="keyword">As</span> Products
<span class="keyword">Set </span>Products =<span class="keyword"> New </span>Products
<span class="keyword">End Property</span></pre>
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<h3>Exe Project</h3>
<ol><li>Select <strong>File > Add Project</strong></li>
<li>Select <strong>Standard EXE</strong></li>
<li>Rename <strong>Project1</strong> to <strong>DBOTest</strong></li>
<li>Right click on<strong> DBOTest </strong>and select <strong>Set As Startup</strong></li>
<li>Select the <strong>DBOTest</strong> project</li>
<li>Select <strong>Project > References</strong></li>
<li>Select the<strong> NorthwindDB</strong> reference</li>
<li>Add a text box to Form1</li>
<li>Set the <strong>MultiLine</strong> property to true</li>
<li>Set the <strong>ScrollBars</strong> property to <strong>2-Vertical</strong></li>
<li>Clear the <strong>Text</strong> property</li>
<li>Paste the following code into Form1:</li>
</ol>
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<td><pre><span class="keyword">
Private Sub</span> Form_Load()<br> <br> <span class="keyword">Dim </span>objProduct <span class="keyword">As</span> Product<br> <br> NorthwindDB.Connect
<span class="keyword">For</span> <span class="keyword">Each</span> objProduct <span class="keyword">In</span> NorthWindDB.Products<br> Me.Text1.Text = Me.Text1.Text & objProduct.Name & " - " & FormatCurrency(objProduct.UnitPrice) & vbCrLf<br><span class="keyword"> Next<br> <br>End Sub</span>
</pre>
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<p>For more
information and examples see the <a href="reference.htm">reference guide</a> and
demonstration program. Both
are available at the website <a href="http://www.hisystems.com.au/databaseobjects">www.hisystems.com.au/databaseobjects</a> in
the downloads section.</p>
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<p align="center"><font size="1">� Hi Integrity Systems 2005. All rights reserved.</font></p>
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