Relation Object

A Relation object represents a relationship  between fields in tables or queries (Microsoft Access database engine databases only).

Remarks

You can use the Relation object to create new relationships and examine existing relationships in your database.

Using a Relation object and its properties, you can:

  • Specify an enforced relationship between fields in base tables  (but not a relationship that involves a query or a linked table).
  • Establish unenforced relationships between any type of table or query— native or linked.
  • Use the Name property to refer to the relationship between the fields in the referenced primary table  and the referencing foreign table .
  • Use the Attributes property to determine whether the relationship between fields in the table is one-to-one  or one-to-many  and how to enforce referential integrity.
  • Use the Attributes property to determine whether the Microsoft Access database engine can perform cascading update and cascading delete operations on primary and foreign tables.
  • Use the Attributes property to determine whether the relationship between fields in the table is left join  or right join .
  • Use the Name property of all Field objects in the Fields collection of a Relation object to set or return the names of the fields in the primary key of the referenced table, or the ForeignName property settings of the Field objects to set or return the names of the fields in the foreign key of the referencing table.

If you make changes that violate the relationships established for the database, a trappable error occurs. If you request cascading update  or cascading delete  operations, the Microsoft Access database engine also modifies the primary key  or foreign key  tables to enforce the relationships you establish.

For example, the Northwind database contains a relationship between an Orders table and a Customers table. The CustomerID field of the Customers table is the primary key, and the CustomerID field of the Orders table is the foreign key. For the Microsoft Access database engine to accept a new record in the Orders table, it searches the Customers table for a match on the CustomerID field of the Orders table. If the Microsoft Access database engine doesn't find a match, it doesn't accept the new record, and a trappable error occurs.

When you enforce referential integrity, a unique index  must already exist for the key field of the referenced table. The Microsoft Access database engine automatically creates an index with the Foreign property set to act as the foreign key in the referencing table.

To create a new Relation object, use the CreateRelation method. To refer to a Relation object in a collection by its ordinal number or by its Name property setting, use any of the following syntax forms:

Relations(0)

Relations("name")

Relations![name]

Example

This example shows how an existing Relation object can control data entry. The procedure attempts to add a record with a deliberately incorrect CategoryID; this triggers the error-handling routine.

Visual Basic for Applications
Sub RelationX()

   Dim dbsNorthwind As Database
   Dim rstProducts As Recordset
   Dim prpLoop As Property
   Dim fldLoop As Field
   Dim errLoop As Error

   Set dbsNorthwind = OpenDatabase("Northwind.mdb")
   Set rstProducts = dbsNorthwind.OpenRecordset("Products")

   ' Print a report showing all the different parts of
   ' the relation and where each part is stored.
   With dbsNorthwind.Relations!CategoriesProducts
      Debug.Print "Properties of " & .Name & " Relation"
      Debug.Print "  Table = " & .Table
      Debug.Print "  ForeignTable = " & .ForeignTable
      Debug.Print "Fields of " & .Name & " Relation"
      With .Fields!CategoryID
         Debug.Print "  " & .Name
         Debug.Print "    Name = " & .Name
         Debug.Print "    ForeignName = " & .ForeignName
      End With
   End With

   ' Attempt to add a record that violates the relation.
   With rstProducts
      .AddNew
      !ProductName = "Trygve's Lutefisk"
      !CategoryID = 10
      On Error GoTo Err_Relation
      .Update
      On Error GoTo 0
      .Close
   End With

   dbsNorthwind.Close
   
   Exit Sub
   
Err_Relation:

   ' Notify user of any errors that result from
   ' the invalid data.
   If DBEngine.Errors.Count > 0 Then
      For Each errLoop In DBEngine.Errors
         MsgBox "Error number: " & errLoop.Number & _
            vbCr & errLoop.Description
      Next errLoop
   End If
   
   Resume Next

End Sub

This example uses the CreateRelation method to create a Relation between the Employees TableDef and a new TableDef called Departments. This example also demonstrates how creating a new Relation will also create any necessary Indexes in the foreign table (the DepartmentsEmployees Index in the Employees table).

Visual Basic for Applications
Sub CreateRelationX()

   Dim dbsNorthwind As Database
   Dim tdfEmployees As TableDef
   Dim tdfNew As TableDef
   Dim idxNew As Index
   Dim relNew As Relation
   Dim idxLoop As Index

   Set dbsNorthwind = OpenDatabase("Northwind.mdb")

   With dbsNorthwind
      ' Add new field to Employees table.
      Set tdfEmployees = .TableDefs!Employees
      tdfEmployees.Fields.Append _
         tdfEmployees.CreateField("DeptID", dbInteger, 2)

      ' Create new Departments table.
      Set tdfNew = .CreateTableDef("Departments")

      With tdfNew
         ' Create and append Field objects to Fields
         ' collection of the new TableDef object.
         .Fields.Append .CreateField("DeptID", dbInteger, 2)
         .Fields.Append .CreateField("DeptName", dbText, 20)

         ' Create Index object for Departments table.
         Set idxNew = .CreateIndex("DeptIDIndex")
         ' Create and append Field object to Fields 
         ' collection of the new Index object.
         idxNew.Fields.Append idxNew.CreateField("DeptID")
         ' The index in the primary table must be Unique in
         ' order to be part of a Relation.
         idxNew.Unique = True
         .Indexes.Append idxNew
      End With

      .TableDefs.Append tdfNew

      ' Create EmployeesDepartments Relation object, using 
      ' the names of the two tables in the relation.
      Set relNew = .CreateRelation("EmployeesDepartments", _
         tdfNew.Name, tdfEmployees.Name, _
         dbRelationUpdateCascade)

      ' Create Field object for the Fields collection of the 
      ' new Relation object. Set the Name and ForeignName 
      ' properties based on the fields to be used for the 
      ' relation.
      relNew.Fields.Append relNew.CreateField("DeptID")
      relNew.Fields!DeptID.ForeignName = "DeptID"
      .Relations.Append relNew

      ' Print report.
      Debug.Print "Properties of " & relNew.Name & _
         " Relation"
      Debug.Print "  Table = " & relNew.Table
      Debug.Print "  ForeignTable = " & _
         relNew.ForeignTable
      Debug.Print "Fields of " & relNew.Name & " Relation"

      With relNew.Fields!DeptID
         Debug.Print "  " & .Name
         Debug.Print "    Name = " & .Name
         Debug.Print "    ForeignName = " & .ForeignName
      End With

      Debug.Print "Indexes in " & tdfEmployees.Name & _
         " TableDef"
      For Each idxLoop In tdfEmployees.Indexes
         Debug.Print "  " & idxLoop.Name & _
            ", Foreign = " & idxLoop.Foreign
      Next idxLoop

      ' Delete new objects because this is a demonstration.
      .Relations.Delete relNew.Name
      .TableDefs.Delete tdfNew.Name
      tdfEmployees.Fields.Delete "DeptID"
      .Close
   End With

End Sub