Storing recordset as instance of a class? - excel

Question

Have the following scenario. I have a few form, which essentially have a few dropboxes, lists etc. I populate them with records from a ms sql db. However, is there a way to query the database only once and store the records as an instance of a class throughout the life of the application rather than querying each time the user opens the form?
Connection is as this:
Sub connection_test()
Dim Cn As ADODB.Connection
Dim Rs As ADODB.Recordset
Dim stSQL As String
stSQL = "SELECT * FROM dbo.Client"
Set Cn = New ADODB.Connection
With Cn
.CursorLocation = adUseClient
.Open CONNECTION_STRING
.CommandTimeout = 0
Set Rs = .Execute(stSQL)
End With
Rs.Close
Cn.Close
Set Rs = Nothing
Set Cn = Nothing
End Sub
Can someone suggest a solution for me? I can see all the problems from heavy traffic and mostly unnecessary.

Answer 1

If you just want the recordset available, dim the variable as public in a standard module
Public Rs As ADODB.Recordset
Sub connection_test()
Dim Cn As ADODB.Connection
Dim sSQL As String
If Rs.State = adStateClosed Then
sSQL = "SELECT * FROM dbo.Client"
Set Cn = New ADODB.Connection
With Cn
.CursorLocation = adUseClient
.Open CONNECTION_STRING
.CommandTimeout = 0
Set Rs = .Execute(sSQL)
End With
End If
End Sub
Now Rs will be available anywhere in the project. You can run connection_test whenever you need to and, if the recordset it closed, it will create it. If not, you're good to go.
Generally my approach to this is to create custom classes. I would create a CClient class, fill it from a recordset (or somewhere else), manipulate the objects with the business logic, then write the new values back to the database. That way, none of my business logic relies on the fact that I'm using ado. I could switch to a text file or an Excel worksheet as a data store and wouldn't have to worry about dependencies everywhere in the code.
For instance, suppose I have an Access table:
ClientID, Autonumber
ContactFirst, String
ContactLast, String
Company, String
CityState, String
Volume, Double
I create a CClient class with a property for each field in my table. I also create a CClients class to hold all of the CClient instances. In a standard module, you might have something like this
Public gclsClients As CClients
Sub Main()
Set gclsClients = New CClients
'Fill the class
gclsClients.FillFromRS
'change some value
gclsClients.Client(1).Volume = 100
'write back to the database
gclsClients.WriteToDB
End Sub
Where I change the volume of one client, you would have a lot more code calling your userform, etc. The basics are, load up the class, do whatever you need, then write the class data back to the db. I won't show you all the class code, but in CClients
Public Sub FillFromRS()
Dim cn As ADODB.Connection
Dim rs As ADODB.Recordset
Dim clsClient As CClient
Const sSQL As String = "SELECT * FROM tblClient"
Set cn = New ADODB.Connection
cn.Open msCON
Set rs = cn.Execute(sSQL)
If Not rs.BOF And Not rs.EOF Then
rs.MoveFirst
Do While Not rs.EOF
Set clsClient = New CClient
With clsClient
.ClientID = rs.Fields("ClientID").Value
.ContactFirst = rs.Fields("ContactFirst").Value
.ContactLast = rs.Fields("ContactLast").Value
.Company = rs.Fields("Company").Value
.CityState = rs.Fields("CityState").Value
.Volume = rs.Fields("Volume").Value
End With
Me.Add clsClient
rs.MoveNext
Loop
End If
End Sub
This method gets the data from the database and fills a bunch of CClient instances. Also in CClients
Public Sub WriteToDB()
Dim cn As ADODB.Connection
Dim rs As ADODB.Recordset
Dim clsClient As CClient
Dim sSQL As String
Set cn = New ADODB.Connection
cn.Open msCON
For Each clsClient In Me
sSQL = BuildUpdateSQL(clsClient)
cn.Execute sSQL
Next clsClient
End Sub
Private Function BuildUpdateSQL(clsClient As CClient)
Dim sReturn As String
With clsClient
sReturn = "UPDATE tblClient SET ContactFirst = '" & .ContactFirst & "',"
sReturn = sReturn & " ContactLast = '" & .ContactLast & "',"
sReturn = sReturn & " Company = '" & .Company & "',"
sReturn = sReturn & " CityState = '" & .CityState & "',"
sReturn = sReturn & " Volume = " & .Volume
sReturn = sReturn & " WHERE ClientID = " & .ClientID & ";"
End With
BuildUpdateSQL = sReturn
End Function
This method loops through all of the CClient instances, creates an UPDATE sql statement and executes it. You'll want to implement some sort of IsDirty property in CClient so that you only update those client where something is changed. The rest of CClients and CClient are basic class module stuff.
You can call WriteToDB a lot or a little. In some apps, I write it whenever something changes. In others, I only write back to the database when the workbook is closed. It kind of depends on the flow of your application. The real beauty is that if you change from, say, an Access database to a text file for data storage, you only have to change to methods in CClients. All of the rest of your code consumes CClients and doesn't care where the data lives.
You can see the workbook and Access database here http://www.dailydoseofexcel.com/excel/ClientClassExample.zip

Answer 2

Read up on Disconnected Recordsets here.
One thing that the article does include (in the sample code), but doesn't emphasize is that you have to use the adLockBatchOptimistic. You do not have to use adOpenForwardOnly, as they do. And, of course, your Recordset object has to have a scope outside the sub. I'd do it this way:
Function connection_test() as ADODB.Recordset
Dim Cn As ADODB.Connection
Dim Rs As ADODB.Recordset
Dim stSQL As String
stSQL = "SELECT * FROM dbo.Client"
Set Cn = New ADODB.Connection
With Cn
.Open CONNECTION_STRING
.CommandTimeout = 0
End With
With Rs
.CursorLocation = adUseClient
.LockType = adLockBatchOptimistic
.Open stSQL
Set .ActiveConnection = Nothing
End With
Set connection_test = Rs
'DON'T Close the Recordset
'Rs.Close
Cn.Close
'Destroying the local instance is fine, though
Set Rs = Nothing
Set Cn = Nothing
End Function
And of course you want to add error handling ;). Then have a Recordset object declared in the calling code, and instantiate it by calling the Function.