How to write in-memory database applications in Lazarus/FPC/ja
English · Français · Русский · 日本語
データベースのポータル
参照: · Databases 概要 · ライブラリ · フィールドの型 · コントロール · FAQ · SqlDBHowto/ja · TSQLQuery の利用 · メモリ上で動作するデータベースの作り方 · チュートリアル/練習となる記事: · 概要 · 0 - データベースのセットアップ · 1 - 開始 · 2 - 編集 · 3 - クエリ · 4 - データモジュール · SQLdb Programming Reference/ja · 各種データベース · Advantage - MySQL - MSSQL - Postgres - Interbase - Firebird - Oracle - ODBC - Paradox - SQLite - dBASE - MS Access - Zeos
イントロダクション
There are certain circumstances when in-memory datasets make sense. If you need a fast, single-user, non mission-critical, non SQL database, without need for transactions, TMemDataset could suit your needs.
Some benefits are:
- Fast execution. Since all processing is done in memory, no data is saved on hard disk until explicitly asked. Memory is surely faster than hard disk.
- No need for external libraries (no .so or .dll files), no need for server installation
- Code is multiplatform and can be compiled on any OS instantly
- Since all programming is done in Lazarus/FPC, such applications are easier for maintenance. Instead of constantly switching from back-end programming to front-end programming, by using MemDatasets you can concentrate on your Pascal code.
https://upload.wikimedia.org/wikipedia/commons/d/d8/Nuvola_apps_ktip.png Note: later on in this article, BufDataset is introduced. TBufDataset often is a better choice than TMemDataset
I will illustrate how to program relational non-SQL memory databases, focusing on enforcing relation integrity and filtering, simulating auto-increment primary fields and similar.
This page shares with you what I have learned experimenting with TMemDatasets. There might be some other, more efficient way to do this. If so, please, feel free to contribute to this document for the benefit of the Lazarus/FPC community.
The memds unit provides TMemDataset, so you will need to add that to your uses clause.
Saving MemDatasets to persistent files
In the interface part of your code, declare an array type for storing information about all the TMemDataSets that you want to make persistent at the end of a session and restore at the beginning of the next session. You have to declare a variable of type TSaveTables, too.
I also use a global variable vSuppressEvents of type boolean, for suppressing Dataset events used for referential integrity enforcement, during data restore.
You get this:
type
TSaveTables=array[1..15] of TMemDataset;
var
//Global variable that holds tables for saving/restoring session
vSaveTables:TSaveTables;
//Suppress events flag variables. Used during data loading from files.
vSuppressEvents:Boolean;Instead of using global variables like I did, you could make them a property of the main form, also. TMemDataset has a way to natively store data to persistent file: the SaveToFile method. But, you could rather choose to save data to CSV files for easier external post processing. Therefore, I will combine both ways into same procedures. I define a constant cSaveRestore in the Interface part, by which I can define whether data will be stored and loaded as native MemDataset files or CSV files.
const
//Constant cSaveRestore determines the way for saving and restoring of MemDatasets to persistent files
cSaveRestore=0; //0=MemDataset native way, 1=saving and restoring from CSVNow, you can save MemDatasets on FormClose event and load them on FormCreate event. Instantiate elements of the array of MemDatasets on the FormCreate event, too.
procedure TMainForm.FormCreate(Sender: TObject);
begin
//List of tables to be saved/restored for a session
vSaveTables[1]:=Products;
vSaveTables[2]:=Boms;
vSaveTables[3]:=Stocks;
vSaveTables[4]:=Orders;
vSaveTables[5]:=BomCalculationProducts;
vSaveTables[6]:=BomCalculationComponents;
vSaveTables[7]:=BomCalculationFooter;
vSaveTables[8]:=BomCalculationProductsMultiple;
vSaveTables[9]:=BomCalculationComponentsMultiple;
vSaveTables[10]:=BomCalculationFooterMultiple;
vSaveTables[11]:=ImportVariants;
vSaveTables[12]:=ImportToTables;
vSaveTables[13]:=ImportToFields;
vSaveTables[14]:=ImportFromTables;
vSaveTables[15]:=ImportFromFields;
//Restore session
RestoreSession;
GetAutoincrementPrimaryFields;
end;procedure TMainForm.FormClose(Sender: TObject; var CloseAction: TCloseAction);
begin
//Save memdatasets to files (to save current session)
SaveSession;
end;procedure RestoreSession;
var
I:Integer;
begin
try
MemoMessages.Append(TimeToStr(Now())+' Starting restoration of previously saved session.');
vSuppressEvents:=True; //Supress events used for referential integrity enforcing
//Disable controls and refresh all datasets
for I:=Low(vSaveTables) to High(vSaveTables) do begin
vSaveTables[I].DisableControls;
vSaveTables[I].Refresh; //Important if dataset was filtered
end;
//Load memdatasets from files (to restore previous session)
for I:=Low(vSaveTables) to High(vSaveTables) do begin
vSaveTables[I].First;
MemoMessages.Append(TimeToStr(Now())+' Starting restoration of table: '+vSaveTables[I].Name);
try
//If data is loaded from a csv file, then table must be deleted first.
if cSaveRestore=1 then begin
MemoMessages.Append(TimeToStr(Now())+' Starting delete of all records in table: '+vSaveTables[I].Name);
//This way of deleting all records is incredibly slow.
{while not vSaveTables[I].EOF do begin
vSaveTables[I].Delete;
end;}
//This method for deleting of all records is much faster
EmptyMemDataSet(vSaveTables[I]);
MemoMessages.Append(TimeToStr(Now())+' All records from table: '+vSaveTables[I].Name+' deleted.');
end;
except
on E:Exception do begin
MemoMessages.Append(TimeToStr(Now())+' Error while deleteing records from table: '+vSaveTables[I].Name +'. '+E.Message);
end;
end;
try
try
MemoMessages.Append(TimeToStr(Now())+' Restoring table: '+vSaveTables[I].Name);
//Check constant for way of saving/restoring data and load saved session
case cSaveRestore of
0:vSaveTables[I].LoadFromFile(vSaveTables[I].Name);
1:LoadFromCsv(vSaveTables[I]);
end;
except
on E:Exception do begin
MemoMessages.Append(TimeToStr(Now())+' Error while restoring table: '+vSaveTables[I].Name +'. '+E.Message);
end;
end;
finally
vSaveTables[I].Active:=True;//Needed because of LoadFromFile method....
end;
MemoMessages.Append(TimeToStr(Now())+' Table: '+vSaveTables[I].Name+' restored.');
end;
finally
vSuppressEvents:=False;
//Refresh all datasets and enable controls
for I:=Low(vSaveTables) to High(vSaveTables) do begin
vSaveTables[I].Refresh; //Needed for tables that are filtered.
vSaveTables[I].EnableControls;
end;
MemoMessages.Append(TimeToStr(Now())+' All tables restored from saved files.');
end;
end;procedure SaveSession;
var
I:Integer;
begin
try
MemoMessages.Append(TimeToStr(Now())+' Starting saving session to persistent files.');
vSuppressEvents:=True;
//Disable controls and refresh all datasets
for I:=Low(vSaveTables) to High(vSaveTables) do begin
vSaveTables[I].DisableControls;
vSaveTables[I].Refresh; //Important if dataset was filtered
end;
//Save session to file
for I:=Low(vSaveTables) to High(vSaveTables) do begin
vSaveTables[I].First;
MemoMessages.Append(TimeToStr(Now())+' Saving table: '+vSaveTables[I].Name);
try
//Check constant for way of saving/restoring data and save session
case cSaveRestore of
0:vSaveTables[I].SaveToFile(vSaveTables[I].Name);
1:SaveToCsv(vSaveTables[I]);
end;
except
on E:Exception do begin
MemoMessages.Append(TimeToStr(Now())+' Error while saving table: '+vSaveTables[I].Name +'. '+E.Message);
end;
end;
MemoMessages.Append(TimeToStr(Now())+' Table: '+vSaveTables[I].Name+' saved.');
end;
finally
vSuppressEvents:=False;
//Refresh all datasets and enable controls
for I:=Low(vSaveTables) to High(vSaveTables) do begin
vSaveTables[I].Refresh; //Needed for tables that are filtered
vSaveTables[I].EnableControls;
end;
MemoMessages.Append(TimeToStr(Now())+' All tables saved to files.');
end;
end;procedure EmptyMemDataSet(DataSet:TMemDataSet);
var
vTemporaryMemDataSet:TMemDataSet;
vFieldDef:TFieldDef;
I:Integer;
begin
try
//Create temporary MemDataSet
vTemporaryMemDataSet:=TMemDataSet.Create(nil);
//Store FieldDefs to Temporary MemDataSet
for I:=0 to DataSet.FieldDefs.Count-1 do begin
vFieldDef:=vTemporaryMemDataSet.FieldDefs.AddFieldDef;
with DataSet.FieldDefs[I] do begin
vFieldDef.Name:=Name;
vFieldDef.DataType:=DataType;
vFieldDef.Size:=Size;
vFieldDef.Required:=Required;
end;
end;
//Clear existing fielddefs
DataSet.Clear;
//Restore fielddefs
DataSet.FieldDefs:=vTemporaryMemDataSet.FieldDefs;
DataSet.Active:=True;
finally
vTemporaryMemDataSet.Clear;
vTemporaryMemDataSet.Free;
end;
end;procedure LoadFromCsv(DataSet:TDataSet);
var
vFieldCount:Integer;
I:Integer;
begin
try
//Assign SdfDataSetTemporary
with SdfDataSetTemporary do begin
Active:=False;
ClearFields;
FileName:=DataSet.Name+'.txt';
FirstLineAsSchema:=True;
Active:=True;
//Determine number of fields
vFieldCount:=FieldDefs.Count;
end;
//Iterate through SdfDataSetTemporary and insert records into MemDataSet
SdfDataSetTemporary.First;
while not SdfDataSetTemporary.EOF do begin
DataSet.Append;
//Iterate through FieldDefs
for I:=0 to vFieldCount-1 do begin
try
DataSet.Fields[I].Value:=SdfDataSetTemporary.Fields[I].Value;
except
on E:Exception do begin
MemoMessages.Append(TimeToStr(Now())+' Error while setting value for field: '
+DataSet.Name+'.'+DataSet.Fields[I].Name +'. '+E.Message);
end;
end;
end;
try
DataSet.Post;
except
on E:Exception do begin
MemoMessages.Append(TimeToStr(Now())+' Error while posting record to table: '
+DataSet.Name+'.'+E.Message);
end;
end;
SdfDataSetTemporary.Next;
end;
finally
SdfDataSetTemporary.Active:=False;
SdfDataSetTemporary.ClearFields;
end;
end;procedure SaveToCsv(DataSet:TDataSet);
var
myFileName:string;
myTextFile: TextFile;
i: integer;
s: string;
begin
myFileName:=DataSet.Name+'.txt';
//create a new file
AssignFile(myTextFile, myFileName);
Rewrite(myTextFile);
s := ''; //initialize empty string
try
//write field names (as column headers)
for i := 0 to DataSet.Fields.Count - 1 do
begin
s := s + Format('%s,', [DataSet.Fields[i].FieldName]);
end;
Writeln(myTextFile, s);
DataSet.First;
//write field values
while not DataSet.Eof do
begin
s := '';
for i := 0 to DataSet.FieldCount - 1 do
begin
//Numerical fields without quotes, string fields with quotes
if ((DataSet.FieldDefs[i].DataType=ftInteger)
or (DataSet.FieldDefs[i].DataType=ftFloat)) then
s := s + Format('%s,', [DataSet.Fields[i].AsString])
else
s := s + Format('"%s",', [DataSet.Fields[i].AsString]);
end;
Writeln(myTextfile, s);
DataSet.Next;
end;
finally
CloseFile(myTextFile);
end;
end;Autoincrement Primary Keys
Autoincrement field type is not supported by MemDataset. Nevertheless, you can imitate it by using Integer field type and providing a calculator for autoincrement fields. We need global variables or public properties for storing current autoincrement field value. I prefer global variables, declared in Interface part.
var
//Global variables used for calculation of autoincrement primary key fields of MemDatasets
vCurrentId:Integer=0;
vProductsId:Integer=0;
vBomsId:Integer=0;
vBomCalculationProductsId:Integer=0;
vBomCalculationComponentsId:Integer=0;
vBomCalculationFooterId:Integer=0;
vBomCalculationProductsMultipleId:Integer=0;
vBomCalculationComponentsMultipleId:Integer=0;
vBomCalculationFooterMultipleId:Integer=0;
vStocksId:Integer=0;
vOrdersId:Integer=0;
vImportVariantsId:Integer=0;
vImportToTablesId:Integer=0;
vImportToFieldsId:Integer=0;
vImportFromTablesId:Integer=0;
vImportFromFieldsId:Integer=0;Then we have a procedure for autoincrement field values calculation:
procedure GetAutoincrementPrimaryFields;
var
I:Integer;
vId:^Integer;
begin
try
MemoMessages.Lines.Append(TimeToStr(Now())+' Getting information about autoincrement fields');
vSuppressEvents:=True;
//Disable controls and refresh all datasets
for I:=Low(vSaveTables) to High(vSaveTables) do begin
vSaveTables[I].DisableControls;
vSaveTables[I].Refresh; //Important if dataset was filtered
end;
for I:=Low(vSaveTables) to High(vSaveTables) do begin
with vSaveTables[I] do begin
//Use appropriate global variable
case StringToCaseSelect(Name,
['Products','Boms','Stocks','Orders',
'BomCalculationProducts','BomCalculationComponents','BomCalculationFooter',
'BomCalculationProductsMultiple','BomCalculationComponentsMultiple','BomCalculationFooterMultiple',
'ImportVariants','ImportToTables','ImportToFields','ImportFromTables','ImportFromFields']) of
0:vId:=@vProductsId;
1:vId:=@vBomsId;
2:vId:=@vStocksId;
3:vId:=@vOrdersId;
4:vId:=@vBomCalculationProductsId;
5:vId:=@vBomCalculationComponentsId;
6:vId:=@vBomCalculationFooterId;
7:vId:=@vBomCalculationProductsMultipleId;
8:vId:=@vBomCalculationComponentsMultipleId;
9:vId:=@vBomCalculationFooterMultipleId;
10:vId:=@vImportVariantsId;
11:vId:=@vImportToTablesId;
12:vId:=@vImportToFieldsId;
13:vId:=@vImportFromTablesId;
14:vId:=@vImportFromFieldsId;
end;
try
//Find last value of Id and save it to global variable
Last;
vCurrentId:=FieldByName(Name+'Id').AsInteger;
if (vCurrentId>vId^) then vId^:=vCurrentId;
finally
//Remove reference;
vId:=nil;
end;
end;
end;
finally
vSuppressEvents:=False;
//Refresh all datasets and enable controls
for I:=Low(vSaveTables) to High(vSaveTables) do begin
vSaveTables[I].Refresh;
vSaveTables[I].EnableControls;
end;
MemoMessages.Lines.Append(TimeToStr(Now())+' Autoincrement fields - done.');
end;
end;function StringToCaseSelect(Selector:string;CaseList:array of string):Integer;
var
cnt: integer;
begin
Result:=-1;
for cnt:=0 to Length(CaseList)-1 do
begin
if CompareText(Selector, CaseList[cnt]) = 0 then
begin
Result:=cnt;
Break;
end;
end;
end;The GetAutoincrementPrimaryFields procedure is called every time after you restore (load) data from persistent files, in order to load last autoincrement values into global variables (or properties, as you prefer). Autoincrementing is done in OnNewRecord event of every MemDataset. For example, for MemDataset Orders:
procedure TMainForm.OrdersNewRecord(DataSet: TDataSet);
begin
if vSuppressEvents=True then Exit;
//Set new autoincrement value
vOrdersId:=vOrdersId+1;
DataSet.FieldByName('OrdersId').AsInteger:=vOrdersId;
end;As already explained, I use vSuppressEvents global variable as flag for the case of restoring data from persistent files.
Enforcing Referential Integrity
There is no enforced referential integrity implemented in MemDataset component, so you have to do it on your own.
Let's assume we have two tables: MasterTable and DetailTable.
There are various places where referential integrity code needs to be used:
- Insert/Update code is located in the
BeforePostevent of the DetailTable: before a new/updated detail record is posted/saved, it needs to be checked for meeting referential integrity requirements - Delete code is located in the
BeforeDeleteevent of the MasterTable: before a master record is deleted, it needs to make sure any child records meet referential integrity requirements
procedure TMainForm.MasterTableBeforeDelete(DataSet: TDataSet);
begin
if vSuppressEvents=True then Exit;
try
DetailTable.DisableControls;
// Enforce referential delete ("cascade delete") for table "MasterTable"
while not DetailTable.EOF do begin
DetailTable.Delete;
end;
DetailTable.Refresh;
finally
DetailTable.EnableControls;
end;
end;procedure TMainForm.DetailTableBeforePost(DataSet: TDataSet);
begin
if vSuppressEvents=True then Exit;
// Enforce referential insert/update for table "DetailTable" with
// foreign key "MasterTableID" linking to
// the MasterTable ID primary key field
DataSet.FieldByName('MasterTableId').AsInteger:=
MasterTable.FieldByName('ID').AsInteger;
end;After you provided referential Insert/Update/Delete, all you must do is provide code for master/detail filtering of data. You do it in the AfterScroll event of the MasterTable and in the OnFilter event of the DetailTable.
Don't forget to set the Filtered property of DetailTable to True.
procedure TMainForm.MasterTableAfterScroll(DataSet: TDataSet);
begin
if vSuppressEvents=True then Exit;
DetailTable.Refresh;
end;procedure TMainForm.DetailTableFilterRecord(DataSet: TDataSet;
var Accept: Boolean);
begin
if vSuppressEvents=True then Exit;
// Show only child fields whose foreign key points to current
// master table record
Accept:=DataSet.FieldByName('MasterTableId').AsInteger=
MasterTable.FieldByName('ID').AsInteger;
end;Known problems
There are several limitations when using MemDatasets.
- Locate method does not work
- Filtering by using Filter and Filtered property does not work. You must use hardcoding in the OnFilter event.
- Looping deletion of records seems to be incredibly slow. Therefore I use my EmptyMemDataset procedure instead of while not EOF do Delete;
- In FPC 2.6.x and earlier, CopyFromDataSet method copies data only from the current cursor position to the end of the source dataset. So, you have to write MemDataset1.First; before MemDataSet2.CopyFromDataSet(MemDataset1);. Fixed in FPC trunk revision 26233.
- Note that older versions of FPC has no CopyFromDataset in Bufdataset, at the time an advantage for MemDs.
- See bug report http://bugs.freepascal.org/view.php?id=25426.
TBufDataSet
As previously mentioned, MemDataSet lacks custom filters, autoincrement data type and the Locate method, so it is better to use TBufDataSet instead. TBufDataset is provided by the BufDataset unit.
Since there is no component for design-time editing of TBufDataSet (but you can set up field definitions at design time), you could create a custom wrapper component or use it through code, in the same way as ClientDataSet in Delphi. Look at the Delphi documentation relating to client datasets for details.
You can use the same methods for enforcing referential integrity and primary autoincrement fields as explained for MemDataSet.
There are only small differences between MemDataSet and BufDataset:
| MemDataSet | BufDataset |
|---|---|
| DataSet.ClearFields | DataSet.Fields.Clear |
| DataSet.CreateTable | DataSet.CreateDataSet |
Sorting DBGrid on TitleClick event for TBufDataSet
If you wish to enable consecutive ascending and descending sorting of a DBGrid showing some data from TBufDataSet, you could use the following method:
Uses
BufDataset, typinfo;
function SortBufDataSet(DataSet: TBufDataSet;const FieldName: String): Boolean;
var
i: Integer;
IndexDefs: TIndexDefs;
IndexName: String;
IndexOptions: TIndexOptions;
Field: TField;
begin
Result := False;
Field := DataSet.Fields.FindField(FieldName);
//If invalid field name, exit.
if Field = nil then Exit;
//if invalid field type, exit.
if {(Field is TObjectField) or} (Field is TBlobField) or
{(Field is TAggregateField) or} (Field is TVariantField)
or (Field is TBinaryField) then Exit;
//Get IndexDefs and IndexName using RTTI
if IsPublishedProp(DataSet, 'IndexDefs') then
IndexDefs := GetObjectProp(DataSet, 'IndexDefs') as TIndexDefs
else
Exit;
if IsPublishedProp(DataSet, 'IndexName') then
IndexName := GetStrProp(DataSet, 'IndexName')
else
Exit;
//Ensure IndexDefs is up-to-date
IndexDefs.Updated:=false; {<<<<---This line is critical as IndexDefs.Update will do nothing on the next sort if it's already true}
IndexDefs.Update;
//If an ascending index is already in use,
//switch to a descending index
if IndexName = FieldName + '__IdxA'
then
begin
IndexName := FieldName + '__IdxD';
IndexOptions := [ixDescending];
end
else
begin
IndexName := FieldName + '__IdxA';
IndexOptions := [];
end;
//Look for existing index
for i := 0 to Pred(IndexDefs.Count) do
begin
if IndexDefs[i].Name = IndexName then
begin
Result := True;
Break
end; //if
end; // for
//If existing index not found, create one
if not Result then
begin
if IndexName=FieldName + '__IdxD' then
DataSet.AddIndex(IndexName, FieldName, IndexOptions, FieldName)
else
DataSet.AddIndex(IndexName, FieldName, IndexOptions);
Result := True;
end; // if not
//Set the index
SetStrProp(DataSet, 'IndexName', IndexName);
end;So, you can call this function from a DBGrid in this way:
procedure TFormMain.DBGridProductsTitleClick(Column: TColumn);
begin
SortBufDataSet(Products, Column.FieldName);
end;Sorting multiple columns in grid
I have written TDBGridHelper for sorting grid by multiple columns while holding shift key. Note MaxIndexesCount must be set quite large for TBufDataSet because there can be quite large combinations of possible sorting options. But I think people would not use more than 10 so setting it 100 should be teoretically Ok.
{ TDBGridHelper }
TDBGridHelper = class helper for TDBGrid
public const
cMaxColCOunt = 3;
private
procedure Interbal_MakeNames(Fields: TStrings; out FieldsList, DescFields: String);
procedure Internal_SetColumnsIcons(Fields: TStrings; AscIdx, DescIdx: Integer);
function Internal_IndexNameExists(IndexDefs: TIndexDefs; IndexName: String): Boolean;
public
procedure Sort(const FieldName: String; AscIdx: Integer = -1; DescIdx: Integer = -1);
procedure ClearSort;
end;
{ TDBGridHelper }
procedure TDBGridHelper.Interbal_MakeNames(Fields: TStrings; out FieldsList, DescFields: String);
var
FldList: TStringList;
DscList: TStringList;
FldDesc, FldName: String;
i: Integer;
begin
if Fields.Count = 0 then
begin
FieldsList := '';
DescFields := '';
Exit;
end;
FldList := TStringList.Create;
DscList := TStringList.Create;
try
FldList.Delimiter := ';';
DscList.Delimiter := ';';
for i := 0 to Fields.Count - 1 do
begin
Fields.GetNameValue(i, FldName, FldDesc);
FldList.Add(FldName);
if FldDesc = 'D' then
DscList.Add(FldName);
end;
FieldsList := FldList.DelimitedText;
DescFields := DscList.DelimitedText;
finally
FldList.Free;
DscList.Free;
end;
end;
procedure TDBGridHelper.Internal_SetColumnsIcons(Fields: TStrings; AscIdx, DescIdx: Integer);
var
i: Integer;
FldDesc: String;
begin
for i := 0 to Self.Columns.Count - 1 do
begin
FldDesc := Fields.Values[Self.Columns[i].Field.FieldName];
if FldDesc = 'A' then
Self.Columns[i].Title.ImageIndex := AscIdx
else
if FldDesc = 'D' then
Self.Columns[i].Title.ImageIndex := DescIdx
else
Self.Columns[i].Title.ImageIndex := -1
end;
end;
function TDBGridHelper.Internal_IndexNameExists(IndexDefs: TIndexDefs; IndexName: String): Boolean;
var
i: Integer;
begin
for i := 0 to IndexDefs.Count - 1 do
begin
if IndexDefs[i].Name = IndexName then
Exit(True)
end;
Result := False
end;
procedure TDBGridHelper.Sort(const FieldName: String; AscIdx: Integer;
DescIdx: Integer);
var
Field: TField;
DataSet: TBufDataset;
IndexDefs: TIndexDefs;
IndexName, Dir, DescFields, FieldsList: String;
Fields: TStringList;
begin
if not Assigned(DataSource.DataSet) or
not DataSource.DataSet.Active or
not (DataSource.DataSet is TBufDataset) then
Exit;
DataSet := DataSource.DataSet as TBufDataset;
Field := DataSet.FieldByName(FieldName);
if (Field is TBlobField) or (Field is TVariantField) or (Field is TBinaryField) then
Exit;
IndexDefs := DataSet.IndexDefs;
IndexName := DataSet.IndexName;
if not IndexDefs.Updated then
IndexDefs.Update;
Fields := TStringList.Create;
try
Fields.DelimitedText := IndexName;
Dir := Fields.Values[FieldName];
if Dir = 'A' then
Dir := 'D'
else
if Dir = 'D' then
Dir := 'A'
else
Dir := 'A';
//If shift is presed then add field to field list
if ssShift in GetKeyShiftState then
begin
Fields.Values[FieldName] := Dir;
//We do not add to sor any more field if total field count exids cMaxColCOunt
if Fields.Count > cMaxColCOunt then
Exit;
end
else
begin
Fields.Clear;
Fields.Values[FieldName] := Dir;
end;
IndexName := Fields.DelimitedText;
if not Internal_IndexNameExists(IndexDefs, IndexName) then
begin
Interbal_MakeNames(Fields, FieldsList, DescFields);
TBufDataset(DataSet).AddIndex(IndexName, FieldsList, [], DescFields, '');
end;
DataSet.IndexName := IndexName;
Internal_SetColumnsIcons(Fields, AscIdx, DescIdx)
finally
Fields.Free;
end;
end;
procedure TDBGridHelper.ClearSort;
var
DataSet: TBufDataset;
Fields: TStringList;
begin
if not Assigned(DataSource.DataSet) or
not DataSource.DataSet.Active or
not (DataSource.DataSet is TBufDataset) then
Exit;
DataSet := DataSource.DataSet as TBufDataset;
DataSet.IndexName := '';
Fields := TStringList.Create;
try
Internal_SetColumnsIcons(Fields, -1, -1)
finally
Fields.Free
end
end;To use sorting you need to call helper methods in OnCellClick and onTitleClick. OnTitleClick - If you hold shift ads new column to sot list ore changes direction to selected column or just sorts one column OnCellClick - If you double click on cell[0, 0] grid clears its sorting
procedure TForm1.grdCountriesCellClick(Column: TColumn);
begin
if not Assigned(Column) then
grdCountries.ClearSort
end;
procedure TForm1.grdCountriesTitleClick(Column: TColumn);
begin
grdCountries.Sort(Column.Field.FieldName, 0, 1);
end;If you have assigned TitleImageList then you can specify which image use for ascending and which for descending operations.
ZMSQL
Another, often better way to write in-memory databases is to use the ZMSQL package:
- http://sourceforge.net/projects/lazarus-ccr/files/zmsql/
- http://www.lazarus.freepascal.org/index.php/topic,13821.30.html
- http://lazarus.freepascal.org/index.php/topic,13821
Contributors
Original text written by: Zlatko Matić (matalab@gmail.com)
Other contributions by contributors as shown in the page History.
From the Free Pascal wiki, page "How to write in-memory database applications in Lazarus/FPC/ja", as the Internet Archive kept it on 2025-01-17, last changed on 2024-03-27. 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.