Change the field value on layer from SQL

MapWinGIS
1

I loading a layer from PostgreSQL. How do I change the field value correctly? The layer is loaded in memory. I don’t need to change the values on the server. I used to do this for DBF files:

var GidrantsLayer: IShapefile = EzMap.Shapefile[GidrantsHandle];
var MwShapeId: integer;
var ShapeGidrant: IShape = GidrantsLayer.Shape[MwShapeId];
GidrantsLayer.StartEditingTable(cBack);
var EzTable: Table = GidrantsLayer.Table;
EzTable.EditCellValue(FieldIndexByName['ID_TYPE_PG'], MwShapeId, 4);
GidrantsLayer.StopEditingTable(True, cBack);
AxMap.Redraw;

Now an exception is called. I’m obviously doing something wrong. However, there is no experience with SQL server.

2

Hello Valeriy.

Layers added from various SQL databases come in as OGR layers, not DBF-based Shapefiles. So when you fetch the layer from the map, you should first get it as an OGR layer, then use GetBuffer to get the layer as a Shapefile. From there, you can do your usual edits, even if you’re not saving back to the SQL source. Here is a simple sample (in VB.NET pseudo-code):

' get the layer from the map as an OGR layer
Dim ogrLayer as OgrLayer = AxMap.OgrLayer(layerIndex)
' now get the Shapefile version of that layer
dim sf as Shapefile = ogrLayer.GetBuffer()
' now that you have a Shapefile reference, you can do your usual edits
If sf.StartEditingShapes() Then
    . . .
End If

Does that help?

3

Hello Jerry.
Yes thanks, this code works. What you need. However, if you use

sf.StartEditingTable(cBack);
sf.StopEditingTable(True, cBack);

then an event occurs access violation in VCRUNTIME140.dll.

And this code
sf.StartEditingShapes()
works perfectly.

I have another question then. And how can these changes be applied to the SQL server?

P.S.
And I also checked my code that I gave at the beginning. It also code works too:

var GidrantsLayer: IShapefile = EzMap.Shapefile[GidrantsHandle]; sf.StartEditingShapes().

It turns out it’s all about this code:

sf.StartEditingTable(cBack);
sf.StopEditingTable(True, cBack);

this code calls access violation.

4

Hello Valeriy.

I will have to set up a quick test to see if I can reproduce the problem. But I’m pretty sure that it is because it is a memory-based Shapefile. If you call Start/StopEditingTable directly, it attempts to update a disk-based shapefile.

Instead you should use the higher-level calls:

sf.StartEditingShapes(True, cBack);
sf.StopEditingShapes(True, True, cBack);

You can use these calls even if you’re not editing the Shapes specifically, and they will handle the differences between memory-based and disk-based saves.

It may well be that there’s a bug if you use them directly. I will look into that.

Thank you,
Jerry.

5

Hello again.

Sorry I never got to your second question. I can provide more details this weekend, but if you haven’t read it yet, you can start with the Detailed Description of OGR layer editing in the helpfile.

Regards,
Jerry.

6

Thank you very much Jerry you’ve been very helpful!