TRYING to keep up with ActiveX Data Objects (ADO)

Size: px

Start display at page:

Download "TRYING to keep up with ActiveX Data Objects (ADO)"

Martha McKenzie
6 years ago
Views:

1 Accessing Records Peter Vogel 2000 Smart Access In this article, Peter Vogel looks at one of the newer features in ADO: the Record object. Peter outlines the object s future and shows how you can use it right now from your Access application to work with files over the Internet. TRYING to keep up with ActiveX Data Objects (ADO) isn t as easy as it was with DAO. ADO is still an evolving technology, with new features being added to the existing ADO objects and whole new objects being added to the ADO package. In this article, I ll review one of the newer objects in the ADO library: the Record object. While the Record object has a bright future ahead of it, there s currently only one ADO provider that supports it. However, using that provider you can retrieve any file, as long as you know the file s URL, or navigate through the directory structure of any site. You can also copy, move, add, or delete any file for which you know the URL. A word of warning: In any application, before taking advantage of any of the newer features of ADO (anything after ADO 2.0), you should make sure that those features will be available on the computer where your application will be installed. While all of the original features of ADO are still available in the latest versions of the technology, the reverse isn t true (see the sidebar Mutating ADO on page 15 for more on this topic). To take advantages of the technology described in this article, you ll need to have ADO 2.5 or later installed on your computer (I actually used ADO 2.6 for my testing). You can download the latest version of ADO, along with a variety of supporting technologies, as part of the Microsoft Data Access Components (MDAC) from Once you ve downloaded and installed it, you ll need to check off Microsoft ActiveX Data Objects 2.x Library in your References list before you can use them. This is required even if you re using Access 2000, as the ADO library that s checked off by default for Access 2000 is 2.1, which doesn t include the Record object. The Record object The Record object sounds like it should be associated with the Recordset object, and, to a certain extent, it is. In theory, the Record object can be used to represent one record in a Recordset. The code to create a Record object from a Recordset object looks like this (where rs is an open Recordset): rc.open rs After this code executes, the Record object rc should then be populated with the current record in the Recordset rs. In practice, for almost every provider that you try, you ll get the message Object or provider is not capable of performing requested operation. The only provider that currently supports the Record object seems to be the Microsoft OLE DB provider for Internet Publishing (also installed with MDAC 2.5 or later). Microsoft s documentation suggests that there are bigger plans for the Record object than this single provider. In the future, Record objects may be used instead of the Recordset object for commands that return a single record. The benefit would be a lighter object (since scrolling wouldn t have to be supported) and faster processing. As yet, though, you can t use the Record object to access standard data sources. The Record object s current purpose in life is to be used with semi-structured resources. A semi-structured resource is a data source that lacks the rigid format of a relational data model. Data in a semi-structured resource is organized (if it s organized at all) as a hierarchy or tree. Hierarchical data sources include file systems and, as in the examples in this article, Web sites. While the Record object documentation suggests that the Record object is ready to be used to process a file system, no indication of how that s to be done is provided. With the Internet Publishing provider, though, you can access any file that you can reach using a URL, security permitting. Opening a Record Opening a Record object looks very much like opening a Recordset: You just pass a command and a connection string to the Open method of the object. The Record object, however, shows a lot of flexibility in the format of the connection string. You can, for instance, specify a provider as you would with a Recordset. To use the Internet Publishing provider to access a site called you d use this text in your connection string: Provider=MSDAIPP.DSO;Data Source= User ID=phv;Password=phvpwd; As a shortcut, the Record object will default to using 14 Smart Access February

the Internet Publishing provider if your connection string contains a URL parameter like this one: rc.open "/","URL=http://www.mserver.

2 the Internet Publishing provider if your connection string contains a URL parameter like this one: rc.open "/","URL= URL= User ID=phv;Password=phvpwd; To use the Internet Publishing provider to retrieve a file called MyFile.txt at the URL you d use this code: rc.open "MyFile.txt","URL= You can t, however, specify both the URL and the Provider property. If you do specify the Provider, then you must specify the URL that you want to access by using the Data Source parameter. You re not restricted to working with individual files. Assuming that you have the appropriate security, you can retrieve a directory from a Web site. This code retrieves the root directory for the mserver site: Figure 1. Component Checker showing the summary report from its analysis. Mutating ADO Much of what you can do with ADO depends on which version of ADO is installed on your computer. And, if you distribute applications, much depends on what you ve installed on your clients computers. So how do you determine what version of ADO you have? The first thing you need to do is discriminate between the version of MDAC (Microsoft Data Access Components) and the version of ADO installed. The version of ADO is relatively easy to determine: Just check the Version property of the Connection object. In Access 2000, you can use the Connection object from the Application object s CurrentProject: Msgbox Application.Connection.Version In earlier versions of Access, you ll need to create a Connection object: Dim conn As Connection Set conn = New Connection Msgbox conn.version The problem is more complicated if you re depending on the rest of the MDAC components the data providers, for instance. Installing MDAC will, of course, upgrade/add/replace all of the components that make up MDAC. However, subsequent installations of other applications (for instance, SQL Server 2000, which installs ADO 2.6) can overwrite portions of the MDAC installation, resulting in a mixture of components that relates to no particular package. In addition, the contents that make up MDAC change over time. The latest version of MDAC, as an example, drops support for the Jet database engine. If your application depends on Jet but the client computer has only ever had the latest version of MDAC installed, you re going to have problems. Given the possible variations, the only safe thing to do when distributing an application that depends on ADO is to include your version of the MDAC components that you need as part of your setup routine. While not a solution that you can use from your application, Microsoft s Component Checker (available at will analyze your installation (see Figure 1) and report on the state of the MDAC components. Component Checker makes a best guess effort to determine what version of MDAC you have installed. The report that Component Checker makes on your version illustrates the difficulty of this effort: On my Windows 2000 Professional installation, it reads The MDAC version that is closest to the version on your computer is 2.5 RTM ( ). Component Checker then lists under the headings File Details, COM Details, and Registry Details all of the problems that it found while analyzing your installation. I ve never seen an installation without problems. My Windows 2000 laptop with ADO installed as part of the operating system had three errors and three warnings. On my Windows 98 desktop computer with MDAC 2.0 installed as part of Visual Studio, I had 11 errors and three warnings. As a test, I did a clean install of MDAC 2.5 on my desktop and reran Component Checker. Component Checker then found six errors and four warnings. Smart Access February

3 The Open method accepts five other parameters in addition to the filename and connection. The third parameter, Mode, determines whether the resource can be updated and whether the resource is opened exclusively. The fourth parameter, CreateOptions, will cause the file or directory to be created if it doesn t already exist. The fifth parameter, Options, controls how and when the data will actually be retrieved (you can defer retrieving the contents of a file while still receiving information about it, for instance). The Options parameter is supposed to support a setting of adopenexecutecommand that would allow you to execute a SQL command to retrieve a single row, but that doesn t appear to be implemented yet. The final two parameters allow you to specify a user name and password. Navigating One of the differences between retrieving a single file and retrieving a directory is that you can retrieve the children of a directory by using the Record object s GetChildren method. The GetChildren method returns a Recordset that lists all of the files and subdirectories for the Record object. This code, for instance, retrieves into a Recordset all of the files and subdirectories of the mserver root directory: Dim rs As Recordset rc.open "/","URL= Set rs = rc.getchildren The Recordset produced by the GetChildren method is a forward-only, server-side Recordset. Because it s a server-side Recordset, it can t be bound to the Recordset property of an Access 2000 Form. If the Record object is updateable, the Recordset will be updateable also. With this Recordset, built using the Internet Publishing provider, you can actually retrieve a Record object from the Recordset as promised in the Microsoft documentation. If you do retrieve a Record from the Recordset, and if the row in the Recordset represents a subdirectory, you can then use the GetChildren method to retrieve all of the subdirectory s files and directories. Navigating through semi-structured information, then, consists of retrieving Recordsets of files and subdirectories and then retrieving the children of the directories. Some terminology is probably appropriate at this stage. All of the items retrieved by GetChildren or the Open method are referred to as nodes. Nodes without children are referred to as leaf nodes and typically contain data (like the contents of a file retrieved from a Web site). Non-leaf nodes (like directories) have other nodes as their content but may also contain data themselves. You can check to see whether a Record object represents a file or a directory (leaf or non-leaf node) by checking the Record object s RecordType property. If that property is set to adsimplerecord, you can t use the GetChildren method. Using GetChildren with a Record of adsimplerecord generates the message Operation is not allowed in this context. If the RecordType property is set to any value other than adsimplerecord, you can at least try to retrieve the node s children. The other three values are adcollectionrecord (a node with children), adrecordunknown (a node of unknown type), and adstructdoc (a COM structured document). The following code cycles through all of the children of the mserver root directory and retrieves the children of those nodes. After retrieving the root directory, the code retrieves a Recordset of all of the root s children by using GetChildren on the root Record. Using standard Recordset processing, the code moves through the rows of that Recordset, opening a Record object for each row and checking its RecordType property. Whenever a nonadsimplerecord is found, the code retrieves a Recordset of the children of that node: Dim rcchild As Record Dim rs As Recordset Dim rschildren As Recordset rc.open "/","URL= Set rs = rc.getchildren Do While Not rs.eof Set rcchild = New Record rcchild.open rs If rcchild.recordtype <> adsimplerecord Then Set rschildren = rcchild.getchildren End If rs.movenext Loop A recursive routine to process all of the directories on the site would look like the following code (and is included in the sample database with this month s Source Code files at Passed a URL, the TraverseSite routine will work through all of the files and directories on the site, displaying the file or directory name: Sub TraverseSite (strurl As String) rc.open "/","URL= & strurl GetChildren rc rc.close Set rc = Nothing End Sub Sub GetChildren (rc As Record) Dim rcchild As Record Dim rs As Recordset Dim rschildren As Recordset Set rs = rc.getchildren Do While Not rs.eof Set rcchild = New Record rcchild.open rs 16 Smart Access February

4 If rcchild.recordtype <> adsimplerecord Then GetChildren rcchild End If rs.movenext Loop End Sub Record fields So what do you do with a Record? Like a Recordset, the first thing that you can do with a Record is access its fields. A Record has a Fields collection that works in the same way as a Recordset. What appears in the Fields collection has more to do with the provider that you use than the data that you access. The Record object guarantees only that two fields will be present in the Fields collection: A field containing the content of the node (for the Internet Publishing provider, that s the content of a file from a Web site) A field containing the absolute URL of the node (for the Internet Publishing provider, that s the URL of the file from the Web site) However, the provider can add any additional fields that it wants to the Record s Fields collection. The Internet Publishing provider puts 18 fields in the Record object s Fields collection, for instance. One of those fields (the third field in the collection, with an index of 2, called RESOURCE_ABSOLUTEPARSENAME) contains the absolute URL. For the file retrieved by this code: rc.open "MyFile.txt", " the RESOURCE_ABSOLUTEPARSENAME field would contain: / MyFile.txt The two fields defined by the Record object aren t guaranteed to appear in the same position for every provider. The Record object provides two enumerated values that you can use to retrieve those two fields, as shown in Table 1. Using adrecordurl, even though it has a value of -2, actually retrieves the third field, RESOURCE_ABSOLUTEPARSENAME, from the Record object s Fields collection. The addefaultstream field doesn t appear in the Record object s Fields collection unless you retrieve it with the enumerated value adrecordurl or an index of -1. Table 1. The Record object s enumerated values for retrieving the two defined fields. Enumerated value Value Field retrieved addefaultstream -1 Node contents adrecordurl -2 Node s absolute URL Continues on page 21 Smart Access February

5 was to warn you that it s harder to work with a chart on a report than on a form. I say that because (are you ready?) the chart s behavior changes. Surprised? So was I! So to spare you some frustration, I ve summarized three of my most disconcerting discoveries in Table 1. You can use my techniques to add professionallooking Gantt charts to your Access (and Excel) applications that will impress your users. I find that getting a chart to look just right is fun and satisfying, and it speaks volumes to my users (at least a thousand words!). The tips and workarounds I ve shared here should inspire you to experiment with the MS Graph library s Chart object in your own applications. GANTT.ZIP at Doug Den Hoed is a founder of Lumina Systems Delivery in Calgary, Canada, which specializes in customized software solutions using Access, Visual Basic, InterDev, SQL Server, and Oracle. Doug drew this article from The KB ( his Access-based commercial package for managing software development projects. doug.denhoed@home.com. Table 1. Chart properties differ on forms and reports. Gotcha Details, advice, and workarounds Property imparity After several hours of denial, I finally confirmed the awful truth: A chart painted on a form has more properties (61) than the exact same chart painted on a report (42). My emphasis is on the differences. If you re counting on changing some properties at runtime on a report, I recommend that you enumerate the properties in design mode (for example, for each prp in objchart.properties debug.print prp.name ), note what s available, and test with Access s /runtime startup option. Can t see/change Rowsource Can t change ChartType One of the main features in my real application lets users change the chart s criteria on the fly. On a form s chart, it was trivial: I changed the chart s Rowsource, and the chart obligingly repainted itself with the new data. On a report s chart, however, I couldn t even see the Rowsource property, let alone set it! I spent several hours superstitiously repainting in different ways to see whether I could get the chart to keep its properties. No luck. I did find a workaround: Bind the chart to a working query (for example, qtmpchart), and manipulate the query s SQL property at runtime. It s a fine line between a cool technique and cheating. The ChartType is another powerful property that disappears when you paint the chart on a report. One workaround I can offer is to paint one chart for each type you might need, and use a Select Case to set the Visible property of the chart that you want to display. This solution has its own problems, as the invisible charts take up resources even if you use separate temporary queries and try to minimize their impact by setting them to impossible conditions (for example, giving the query a WHERE clause of 1=2). Ultimately, I abandoned this approach and created a separate report for each chart type. Accessing Records... Continued from page 17 The other fields provided by the Internet Publishing provider include the name of the parent directory (RESOURCE_PARENTNAME) and flags indicating whether the file is hidden or read-only, among other information. You can add new fields to the collection just by referring to them. For instance, this code adds a new field called Handled to a Record s Fields collection and sets it to True: rc!handled = True The field that s created will have the Variant data type, which Microsoft s documentation says isn t yet supported by ADO. The fields that are added are only temporary unless you use the Fields collection s Update method, at which point, presumably, the underlying physical entity will be updated with the new field and its content. The Internet Publishing provider won t update a file or directory, so it will only let you add fields temporarily. Calling the Update method of the Fields collection for the Internet Publishing provider will generate the message Current Provider does not support adding and deleting columns on the Record object. Working with files In addition to browsing the structure of the Web site and retrieving files, you can use the Record object to change the files on the site by using the Record object s CopyRecord, DeleteRecord, and MoveRecord methods. The DeleteRecord method is the simplest. This method, used without a parameter, deletes the resource specified by the Record object. This code deletes the physical entity pointed to by the Record object: rc.deleterecord Smart Access February

6 The DeleteRecord method also accepts a parameter that specifies the resource to be deleted. The MoveRecord method copies the resource to the location specified by the second parameter and then deletes the resource. If the first parameter is omitted, the MoveRecord method copies the resource that s pointed to by the Record object. If the first parameter is provided, that s the resource that will be copied. The MoveRecord method also accepts as its third and fourth parameters a user name and password to access the location where the resource will be copied. The fifth parameter to the method controls what happens if the target of the move already exists. The CopyRecord method acts like MoveRecord but doesn t delete the resource after making the new version. Many of the methods of the Record object have an Async parameter that allows you to execute the method asynchronously. However, the Record object doesn t fire events, so you ll have to monitor the progress of the methods by checking the Record object s State property. Manipulating Internet files can be useful, but what you really want to do is get at the content of those files that you re retrieving. The field retrieved with the addefaultstream value is actually a Stream object, another of the new ADO objects added in ADO 2.5. Next month, I ll show you how the Stream object works and how to integrate it both with XML and the Record object. ADORCRD.ZIP at Peter Vogel (MBA, MCSD) is the editor of Smart Access and a principal in PH&V Information Services. PH&V specializes in system design and development for COM/COM+ based systems. Peter has designed, built, and installed intranet and component-based systems for Bayer AG, Exxon, Christie Digital, and the Canadian Imperial Bank of Commerce. He s also the editor of Pinnacle s XML Developer newsletter, wrote The Visual Basic Object and Component Handbook (Prentice Hall), and is currently working on a book on user interface design (Apress). Peter teaches for Learning Tree International, wrote its Web application development course, is technical editor of its COM+ course, and is currently developing its Technical Writing course. His articles have appeared in every major magazine devoted to VB-based development and in the Microsoft Developer Network libraries. Peter also presents at conferences in North America and Europe. peter.vogel@phvis.com. XML Web Development SQL Server Visual Basic MS Access Oracle Visual C++ Delphi FoxPro XML Web Development SQL Server Visual Basic MS Access Oracle Sign up now for Pinnacle s FREE enewsletters! Get tips, tutorials, and news from gurus in the field delivered straight to your Inbox. XML Web Development SQL Server Visual Basic MS Access Oracle Visual C++ Delphi FoxPro XML Web Development SQL Server Visual Basic MS Access Oracle Downloads February 2001 Source Code DTSHEETS.ZIP Michael Kaplan s sample database includes a number of routines for manipulating datasheets. The code in this MDB file can be used to show and hide columns, prevent your users from repositioning columns, and resize columns to fit the data that they contain. (Access 97, Access 2000) IDENTITY.ZIP This Access add-in not only includes all of Russell Sinclair s code for working with AutoNumber fields, it also includes a user interface that allows you to modify the AutoNumber fields in your MDB file. (Access 2000) ADORCRD.ZIP Peter Vogel has provided a sample database that shows how to use the ADO Recordset object. The routines TraverseSite and GetChildren will provide a complete file listing for any Web site for which you have the URL (security permitting). (Access 97) GANTT.ZIP In his sample database, Doug Den Hoed has included all of the code and forms to generate Gantt charts from Access data. You ll need to have MS Chart installed to use this code. Doug has also provided the sample Excel spreadsheet that he used in developing his code. (Access 97) SA0102.ZIP and SA0102.CHM The latest update to the cumulative Smart Access index comes in Access 2.0, Access 95, and Access 97 versions. It includes full descriptions of every article since the January 1999 issue of Smart Access, as well as an index to each month s Source Code files. (The index is available in SA0201.ZIP The Complete February 2001 Source Code. ) 22 Smart Access February

Course 319 Supplementary Materials. Effective User Manuals

Course 319 Course 319 Supplementary Materials www.learningtree.com 1 Course 319 Originally published in the April, 2000 issue of the Smart Access newsletter from Pinnacle Publishing (www.pinpub.com). Peter