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Associating WMI InstancesHopefully, my previous two examples make it easier for you to understand WMI associations. However, they're bad examples for truly working with associated classes. Why? Because you aren't ever going to begin knowing which instance of the associator class you want; you're going to begin with one of the associated classes instead. Using the preceding technique, suppose you want to find the shares for a particular printer. You'd have to
ASSOCIATORS OFThe aforementioned technique is an awkward way to get the information, and that's why WQL offers the ASSOCIATORS OF command. Suppose you have a printer with a device ID of "LaserJet 5." You've created three or four shares of the printer, each with different permissions or whatever. You want to use WMI to retrieve the name of each share, and list the maximum concurrent number of users allowed to use each share. You could write a WQL query like this: ASSOCIATORS OF {Win32_Printer.DeviceID = "LaserJet 5"}. Note that ASSOCIATORS OF replaces the SELECT, property list, FROM, and class name elements of a more traditional WQL query. Also note that the class must be listed in curly braces {} not parentheses. That messes me up every time. Figure 19.3 shows the results of this query in Wbemtest (assuming you have a printer named LaserJet 5, that is; for this example, I used a different printer name). Figure 19.3. Results of the ASSOCIATORS OF query
It turns out there are several associated classes:
You can restrict the list just to the Win32_Share class by modifying the query a bit.
ASSOCIATORS OF {Win32_Printer.DeviceID = "LaserJet 5"}
WHERE AssocClass = Win32_PrinterShare
This modified query just returns an instance of Win32_Share for each share that exists for the printer. Note that the query does not return Win32_PrinterShare instances; WMI is smart enough to know that although Win32_PrinterShare is the associator class, you're really after the other side of the relationship, which is Win32_Share. The following script displays each share name for the specified printer, and the number of connections each accepts.
Dim oWMI, oPShares, oPShare
Dim oShare
'connect to WMI
Set oWMI = GetObject("winmgmts:\\.\root\cimv2")
'retrieve all settings
Set oPShares = oWMI.ExecQuery("ASSOCIATORS OF {Win32_Printer.DeviceID='LaserJet 5'} WHERE AssocClass = Win32_PrinterShare")
For Each oShare in oPShares
'display share info
If oShare.AllowMaximum = False Then
WScript.Echo "Share " & oShare.Name & _
" allows " & oShare.MaximumAllowed & _
" concurrent connections."
Else
WScript.Echo "Share " & oShare.Name & _
" allows max connections."
End If
Next
REFERENCES OFThe WQL REFERENCES OF query works similarly to ASSOCIATORS OF. It's designed to return all association instances that refer to a specified source instance. However, whereas ASSOCIATORS OF attempts to retrieve the endpoint instances (such as mapping Win32_Printer all the way through to Win32_Share), REFERENCES OF only attempts to find the associator classes (such as Win32_PrinterShare). For example, use Wbetmtest to execute the following query.
REFERENCES OF
{Win32_Printer.DeviceID = 'printername'}
Of course, replace printername with a valid printer on your computer. NOTE Remember, to execute a query in Wbemtest, first connect to the \default\cimv2 namespace. Then, click the Query button and type the query into the text box. The query returns several classes:
These are all of the associator classes that refer to the specified Win32_Printer instance. If you want to get the endpoint of the association-in other words, the actual driver, printer setting, device, share, or driver DLL-you'd use ASSOCIATORS OF instead. I don't find much need for REFERENCES OF in my administrative scripts, because I'm usually looking for the other end of the association, not the middle point. Using WHERE with ASSOCIATIONS OF and REFERENCES OFBoth the ASSOCIATIONS OF and REFERENCES OF queries support a WHERE clause; I showed you an example using the AssocClass keyword earlier. REFERENCES OF accepts the following keywords in its optional WHERE clause.
ASSOCIATORS OF supports different WHERE options.
All of these WHERE clause keywords can be combined (except as I've noted here), and do not require commas or any other separation. For example:
ASSOCIATORS OF {Win32_Printer.DeviceID = 'LaserJet5'}
WHERE
ResultClass = Win32_Share
RequiredQualifier = AllowMaximum
Don't be tempted to include an AND keyword like you would in a traditional WHERE clause, because WMI will return an error. |
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