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We ll discuss concurrency in chapter 8. For now, understand that when making updates, we check to see if values were changed since we last fetched the values. Because the database updates a timestamp each time the row is changed, we don t need to worry about changes to any other column. By combining the previous value of the ID and timestamp, we check to see that someone else didn t make a conflicting change to our record while we were working on it. The rest of the properties are not needed for concurrency checking and thus we can state that we never need to check the values on update (UpdateCheck.Never). With these mappings in place, we re ready to perform standard queries against our revised Author class. If we wanted to work with object trees to join our authors with the books using the AuthorBooks table, we would need to specify one more set of mapping attributes, Association. Association attribute The Association attribute is used to designate how two classes, and by extension their corresponding tables, are related. Unlike the Table and Column attributes, at least one parameter is required in order for our association to work. Table 7.3 lists the parameters used by the Association attribute.

Option Compare Database Option Explicit Dim WithEvents frmWedding As Form_WeddingListForm End of the Declarations Section

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Private Sub Form_Load() On Error GoTo Form_Load_Err If the wedding list form is open If IsLoaded("WeddingListForm") Then Then set to respond to the NewCity event Set frmWedding = Forms!WeddingListForm End If Form_Load_Exit: Exit Sub Form_Load_Err: MsgBox Error$ Resume Form_Load_Exit End Sub Private Sub frmWedding_NewCity(varCityName As Variant) The Wedding List form has asked us to move to a new city via the NewCity event On Error Resume Next If IsNothing(varCityName) Then Hide me if city name is empty Me.Visible = False Else Reveal me if there s a city name, and go find it Me.Visible = True Me.Recordset.FindFirst "[CityName] = """ & _ varCityName & """" End If End Sub

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In the Declarations section, you can find an object variable called frmWedding that has a data type equal to the class module name of WeddingListForm. The WithEvents keyword indicates that code in this class module will respond to events signaled by any object assigned to this variable. When the form opens, the Form_Load procedure checks to see that WeddingListForm is open (just in case you opened this form by itself from the Database window). If WeddingListForm is open, it hooks the NewCity event in that form by assigning it to the frmWedding variable.

Table 7.3 Listing of parameters used by the Association attribute Description Indicates the cascading delete policy for the relationship. Used in 1:1 relationships to indicate the cascading delete policy when the foreign key fields are not nullable. Indicates that the class in question is the child of a parent-child relationship. Used to indicate a 1:1 relationship where both the foreign key and primary key are unique and contained in both tables. This is not used often, as most relationships are 1:0-1 or 1:n rather than a true 1:1. Specifies the name of the foreign key that will be used when dynamically creating the database from the metadata. Used to identify the column(s) in the associated class that contain the related key values. If the parameter is not specified, the ID columns specified in the other class will be assumed. Specifies the internal field used to track the related child object EntitySets. Identifies the property that contains the local ID field. If this is not specified, the column(s) designated by IsPrimary in the Column attribute are used. If the key consists of multiple columns, include each of them in a comma-separated list of the column names.

Automating Your Application with Visual Basic The frmWedding_NewCity procedure responds to the NewCity event of the frmWedding object. Once the Load event code establishes frmWedding as a pointer to WeddingListForm, this procedure runs whenever code in the class module for that form signals the NewCity event with RaiseEvent. The code in the event procedure is pretty simple. If the CityName parameter passed by the event is nothing (Null or a zero-length string), the procedure hides the form because there s nothing to display. If the event passes a valid city name, the procedure uses the FindFirst method of the Recordset object of this form to move to the correct city. Note The Recordset property of a form in an Access database (.mdb file) returns a DAO recordset in Access 2003. For this reason, you should use a DAO FindFirst method, not an ADO Find method, to locate rows in a form recordset.

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The most complex Visual Basic code we ve examined thus far in this chapter is the procedure to build a search clause from the data you enter in the fdlgContactSearch form. Trust me, we ve only started to scratch the surface!

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Given this information, let s take a look at how we can add an association between our new Author class and a BookAuthor class.

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