The Access Wizard
The Access Wizard Newsletter Tips Tricks and Traps for Access Users and Developers
October 2005

Combo Boxes 102 – Building from Scratch

I love home-baked bread. About fifteen years ago I actually tried to make some. Although it was an adventure, the bread was dense and not very tasty. About five years back, I discovered bread machines, and boy was I in heaven. Dump the ingredients in, turn on the machine, wait a couple of hours, and voila---tasty warm bread. My family enjoyed it, too.

Although I was pleased with the bread from the machine, it got a bit monotonous. I could vary the ingredients, but I really couldn't easily control the process or the shape of the final product. Last winter, I took a bread making course at the local high school. My eyes were opened to what real home- baked bread was all about. The instructor taught us about kneading, rising, yeast and other leavening agents, shapes, ingredients, baking temperature, and crusts (use a water spritzer in the oven before you put the bread in to make it nice and crispy). My knowledge and ability to turn out bread, just the way I wanted it, was light years beyond what I was able to accomplish with a bread machine.

Because of that course, I can now make much better bread to suit my mood and the preference of our guests (if you ever come over to the house ask for the soft dough rolls – they're a winner.)

Last month we learned how to create a combo box by using a wizard, a process very similar to using a bread machine. We get a good product in the end, but the amount of control and the ability to get exactly what we want is severely limited. This month we do the equivalent of moving from machine-made bread to hand-made bread. We'll get exactly what we want by controlling the process from beginning to end – combo boxes from scratch!

In this issue
  • Tip of the Month – There's More than One Way to Skin a Cat
  • The Starting Point
  • Combo Box Data Details
  • Getting What we Really Want
  • Trap of the Month – Don't Use Existing Queries as Sources for Combo Boxes

  • The Starting Point

    Let's assume that we have the following table containing campers at a fictional summer camp called Camp Sunapee (it may look familiar because we’ve used it in past newsletters):

    Let's say we want to build a combo (drop down) box showing each camper in the format last name, first name. What we want to get back when a user selects the camper is the camper key as shown in the first column. The idea here is that we'll store and manipulate the camper key, a unique identifier, in other parts of the program (and in future Access Wizard newsletters). We’ll be able to use the camper key to extract information of interest related to each camper. This would not be an easy thing to do in a wizard since we're not given the option to concatenate fields (like last name, first name) during the creation of a combo box. By creating the combo box manually, we'll be able to create the exact combo box we want.

    Let's start with a blank form by clicking on the forms tab, choose create form in design view, choose design view again, and then click OK. Then select combo box from the toolbox (for details see September 2005).

    When you double click on the combo box on the toolbar (see the illustration below (200509Toolbox.jpg)

    and then draw a rectangle on the blank form you'll get the same dialog box we got last month. This time, however, choose cancel so we can manipulate the combo box manually rather than be restricted by the choices the wizard offers.

    Combo Box Data Details

    Right click the combo box and choose properties. You'll get the following dialog box:

    There are a number of tabs that we'll be delving into in the coming months, each of which focus on certain aspects and properties. For now we'll stick with the data tab and concentrate on the properties related to data and to what the user sees.

    Two important attributes that influence both what the user sees and the functionality of a combo box are the Row Source Type and Row Source. The row source type has three possible choices: Field List, Value List, and the default Table/Query.

    Let’s explore what these Row Source Type choices can do for you as you build your custom combo box:

    • Field List: this is a list of field names from a table, query, or SQL statement specified by the RowSource setting. This choice has limited value or flexibility when designing a custom combo box.
    • Value List: used with a predefined static list of no more than about a dozen choices (e.g. male/female, yes/no, cash/check/credit, or a limited list of cabins at the camp). To enter a value list, simply select Value List as the row source type and then in the row source area type in each of values separated by commas. I have to say I'm not a fan of Value Lists because changing the values down the road is a manual process.
    • Table/Query is where the real action is when building custom combo boxes! This choice allows the maximum control and formatting over what you show your users. It's also dynamic, which means that as values are added and removed from tables the items in the list change and grow according to what is in the underlying tables. Since this is the default for Row Source type we'll leave that as it is.

    If you click in row source after selecting Table/Query as the row source type you'll get two possibilities: 1) the downward facing arrow and 2) the ellipsis (...). If you choose the downward arrow you'll be presented with a list of tables and existing queries – don't choose that because it will limit your ability to control what you show your users. Instead, click on the ellipsis and you be presented with the same dialog box you get when you start a new query from scratch. In our case we'll choose the camper table, and from here you are in your regular Query by Example grid. We'll make our selections and build our query as follows:

    Notice that I've selected the primary key as the first column, and then concatenated the last and first names (See the June 2005 Access Wizard to get the details on how to do this). Finally, we've set the sort order on the Camper name. When you run this, and I always encourage many small tests along the way, you get the following:

    The important things to notice here are that the primary key is in the first column and the complete camper name (in concatinated form) is in the second column. If you wanted to you could also add other fields to the right to provide more information to your users.

    When you close this query you'll be asked if you want to save it. Say yes and you'll be bounced back to the form in design view.

    To run the form, select View | Form View and click on the downward black triangle and you'll see:

    Getting What we Really Want

    The picture above shows a series of numbers -- not terribly helpful. What's going on here is that the combo box is showing the first column from the query you just built. It's easy, however, to present results to your users that make more sense. Go back to design view for the form (View | Design View), select the combo box, and open the properties window. This time click on the format tab and make two changes:

    1. Change the column count to 2. The column count is the property that tells Access how many columns the combo box should deal with. We're choosing 2 because we want to hold onto the primary key, column 1, yet show our users the second column, the one that contains the camper name. This number can be altered to reflect your particular situation.
    2. Change the column width property of each column displayed. Since our first column holds the primary key, showing this to the user will do nothing but confuse him. To hide it, simply set that column width to 0, followed by a comma and the width of subsequent columns expressed in inches (you can use decimals to get fractional inches if you want). In this case, we set the width for column 2 to one inch, wide enough to display the camper names.

    When we run this we get:

    This is what we've been aiming for all along: a list of our campers, showing both first and last name, alphabetized by last name.

    Throughout this issue, we’ve made an effort to preserve the primary key as a field in the combo box. In coming months, you’ll learn more about why we’ve done this as we dive deeper into combo boxes.

    Trap of the Month – Don't Use Existing Queries as Sources for Combo Boxes

    Don't use existing queries as the source for combo boxes. If you do, you're asking for trouble. The issue here is that if you use an existing query, it's difficult to tell where else in your application it might be used. If at some future point you change the query, for example changing the order of columns, changing the sort order, or even deleting the query altogether, your combo box will not work as intended. It’s likely that you will get one of those nasty error messages and not understand why. The same holds true for saving your query if you create the combo box. When you create a query for a combo box, don’t save it. If you give it a name, it will continue to show up in the list of all queries in your application, leaving it vulnerable to potential changes.

    The best way to deal with this is to simply build the query using the ellipsis (...) from the property box and then just close the query when you're finished. Access will ask you if you want to save the changes. Say yes and the query will be saved along with combo box, but will not be exposed in the general query window. This seems confusing, and frankly it is. When Access asks you if you want to save the changes, it's really asking if you want to preserve the changes you make. Explicitly saving the changes however, will require you to give the query a name which means it will then show up in your query list. I know this seems not to make sense, its one of those things that its best to just live with.

    If you save the query only as you're closing the query window and you ever need to make a change, just open the properties of the combo box, select row source, make your changes and close the query, responding yes to save the changes.

    Tip of the Month – There's More than One Way to Skin a Cat

    There are many ways to get property information for an entity. This month when we wanted to manipulate the properties of a combo box, I instructed you to right click on the combo box and choose properties. There are two other ways you could have gotten there.

    The first is to click on view from the menu at the top of the screen and choose properties.

    If you like keyboarding your way around Access rather than mousing, there's a nice alternative. You can use the Alt-Enter keyboard combination. In either of these cases, you have to make sure that the combo box or whatever you happen to be interested in is selected before you choose the menu item or keyboard shortcut.

    If you really like keyboarding and shortcuts check out the May 2005 issue of the Access Wizard. I go into detail there on some shortcut tactics and give you some of my favorite ways to speed your way around Access

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