TDBLookupComboBox: what changed

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Title: TDBLookupComboBox

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1	+	TDBLookupComboBox ![](/wiki-images/4/46/Tdblookupcombobox.png) is a (doubly) data-bound [[TComboBox]]. A TDBLookupComboBox can be found on the [[Data Controls tab]] of the [[Component Palette]].
2	+
3	+	The TDBLookupCombobox control gets a list of values from its ListSource (which e.g. represents a table with product info, "Products"). It then:
4	+
5	+	- displays the values in the ListField (e.g. a "ProductName" field) while
6	+	- remembering the values in the KeyField (e.g. an "ID" field)
7	+
8	+	The combobox stores the result (i.e. the KeyField value in the DataField, e.g. the "ProductID" field in the DataSource property (e.g. an "Orders" table).
9	+
10	+	The difference with the [[TDBComboBox]] is that the TDBComboBox is a (as it were) "singly data bound control": it stores the results in a database field but the list of values to select from is supplied by code/via the [[Object Inspector]].
11	+
12	+	## Unbound use
13	+
14	+	You can get the combobox to look up values from one table without storing the results in another by leaving the DataSource and the KeyField properties empty.
15	+
16	+	The following sample code snippet will:
17	+
18	+	- Allow a TDBLookupCombo box to be configured in unbound mode (i.e. no changes made to any table in the database)
19	+	- Ensure the visible data in the TDBLookupComboBox is from the selected record in the dataset (i.e. choosing a new value in TDBLookupComboBox contents will scroll the dataset to the correct record)
20	+	- Set the initial displayed value in the TDBLookupComboBox and the selected record in the datset to previously remembered values.
21	+
22	+	The following sample code snipper will NOT:
23	+
24	+	- Automatically update the TDBLookupComboBox value if the dataset is scrolled (you need to add your own handler to Dataset.OnScroll to achieve this)
25	+
26	+	``` pascal
27	+	interface
28	+	type
29	+	TForm1 = class(TForm)
30	+	cboLookup: TDBLookupComboBox;
31	+	dsSource: TDatasource;
32	+	dsetSource: TDataset;
33	+	...
34	+	protected
35	+	FDisplayField: string;
36	+	FKeyField: string;
37	+	FPreviousKeyValue: string; // May be any datatype - see below
38	+	...
39	+	end;
40	+
41	+	...
42	+
43	+	implementation
44	+
45	+	...
46	+	// Remember previous value
47	+	FPreviousKeyValue:= load_from_settings_in_your_preferred_way_;
48	+
49	+	...
50	+	// Configure DBLookupCombo
51	+	dsSource.Dataset := dsetSource;
52	+	cboLookup.ListSource := dsSource;
53	+
54	+	cboLookup.ScrollListDataset := True; // This ensures that changing the ComboBox will automatically scroll
55	+	// the dataset
56	+	cboLookup.Style := csDropDownList;
57	+
58	+	cboLookup.KeyField := FKeyField;
59	+	cboLookup.ListField := FDisplayField; // This is the field that will appear in the contents of the ComboBox
60	+
61	+	dsetSource.Open; // The ComboBox should now be populated, however intially no text is displayed
62	+	// So right now there is no relationship between the selected record in the dataset
63	+	// and the contents of the ComboBox
64	+
65	+	// cboLookup.KeyValue reads and writes a Variant, so you may actually define
66	+	// FPreviousKeyValue as any data type you feel is appropriate.
67	+
68	+	// Remember a previous value
69	+	if FPreviousKeyValue <> '' then
70	+	begin
71	+	dsetSource.Locate(FKeyField, FPreviousKeyValue, []); // Note: This will fail to correctly set the ComboBox,
72	+	// But will set the Datatset to the correct record.
73	+
74	+	// Now the dataset is on the correct record, lets ensure the DBLookupCombo
75	+	// is showing the same value as the selected record
76	+	If (cboLookup.KeyValue = Null) And (dsetSource.RecordCount > 0) Then
77	+	cboLookup.KeyValue := dsetSource.FieldByName(FKeyField).AsVariant;
78	+	end;
79	+
80	+	...
81	+	// Save current value for next time
82	+	FPreviousKeyValue:= cboLookup.KeyValue;
83	+	write_to_settings_in_your_preferred_way_(FPreviousValue);
84	+	```
85	+
86	+	## Bugs
87	+
88	+	At least for TDBLookupComboBox, there is a bug with FPC 2.6.0 (used in Lazarus 1.0, 1.0.2 etc) that requires the listfield to be present in the datasource as well.
89	+
90	+	Workaround: you can bypass this by declaring a calculated field with the same name as the listfield in the datasource's dataset that does nothing.
91	+
92	+	## Alternative control
93	+
94	+	The Rx controls (in the RxNew package) have the RxDBLookupCombobox which has some extra functionality; it e.g. allows to display multiple fields/columns next to each other (much like MS Access comboboxes):
95	+
96	+	``` pascal
97	+	RxDBLookupCombo.LookupDisplay = 'field1;field2'; //takes a semicolon-delimited list of fields
98	+	```
99	+
100	+	It additionally has a property DisplayAllFields.
101	+
102	+	## See also
103	+
104	+	- [[doc:lcl/dbctrls/tdblookupcombobox.html\|TDBLookupComboBox doc]]
105	+	- [[TComboBox]]
106	+	- [[TDBComboBox]]
107	+
108	+	---
109	+	From the Free Pascal wiki, page "TDBLookupComboBox", as it stood on 2020-02-29 after a change by Trev, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).

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