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#21: DB programming technology (Design Patterns)

Today I answer in Delphi-DB mailing list about subj. I remember, that few weeks ago we too wanted to discuss this theme. Also I decided to post this message here too. Let it will be by today's delphi tip.

On server database I have a few own "system" tables with descriptions of project:
1. CLASS table is a list of all available user queries with short description, select/update/insert/delete statements
2. DESCRIPTION table stores the descriptions of fields for each record in CLASS
3. a few additional tables with security (user, groups, privileges, access modes etc)
PS I do not describe them, as for the basic description they are not so important

In application I have a main datamodule, which contains the one TDatabase component (linked to my alias).
Also I have a some TDataModule component which are the parent for each other. On this component I dropped the TQuery (linked to main TDataBase), and TUpdateSQL and TDataSource, which are linked to TQuery.

Also in this parent datamodule I included the some additional properties and procedures/functions:
1. to get a descriptions from server by class name
2. to fill a update/delete/insert properties of TUpdateSQL
3. to change a filter (via where clause), a data order (via order by) etc (any clause in sql - I use a some macros in own sqls)
4. save/cancel changes, refresh data
5. to get the new ID (for autoincremental fields)
6. to read a user privileges
7. to open class
8. to get a "lookup" class
PS: I not use a delphi lookup fields. Instead it I store in class descriptions for some fields the additional info (which class I must open for edit/view of data, which fields are linked in "lookup" class etc and more)
9. and more additional features

Each datamodule are successor of this parent DM with "personal" extension in business logic. For example, in the class for details of orders I defined the specific calculations (some subtotals and sums) or procedures (check of outputing qnt). The full logic I included in these DMs only (in visual forms I use a calls only!).

Also I have a basic visual TForm with grid, navigator, filter panel etc.
This form have a "linked" DataModule. Also this form known how:
1. to fill the info from DM (a columns list for dbgrid (from description in DM), form caption and more)
2. to call a form for edit/append of record (those form not uses the any DB-components!!)
3. to generate report (I use a SMReport)
4. to export data (I use a SMExport)
5. to find a record by some criteria
6. to select a records (the some forms I use for selecting of some recors)
PS: for example, in orders mode my users can select a records from product class and append the wished

7. to open a "lookup" class
PS: for example, in grid with customer list my users can open a class of customer types (instead standard delphi lookup field) or "drill" in orders list for this customer

8. to setup the visualization of data (fonts, colors, order etc, filter, sort and grouping)
9. and more additional features

All the rest forms are successors of this parent and are different one from other in needed possibilities only. For example, if my users want to calculate a sum of some order for specific product type only, of course, I add a calc function in linked DM and drop a button on form for calling of this function in DM

PS: in visual forms (when I need to calc or to do a something) I call the "assigned" method in "assigned" DM.

Also I have a two list - opened DMs and created forms. So if I need open a "lookup" class, I search it in list of DM and create only if I not found. And with forms I work similar.

PS: this technology I uses in tens projects (current and finished), in small app and in big projects. I described the customers-orders-products schema for availability of understanding only. Of course, sometimes (more often in the beginning of development of small app) the perspective of the extension of functionality up to large client-server system (and especially multi-tier) is not visible and it's possible to go on easy way - drop on form the ttables (or even to allocate them in separate datamodule), linked it with grids, use a master-detail link and lookup fields etc
But when you decide to expand possibilities of the app and transfer it from local DB (Paradox/DBase/Access etc) on normal DB server and maybe use a 3-tier you will understand, that is necessary to change the approach to DB programming for rise of productivity, easy support and extension of project.

Of course, it's my opinion only, but I have come to such technology by many cuts and tries during 10 years on different DBs and tools of development. Though I still do not have also thirty years I have a large number of successful developments in other departs and if I can to someone reduce this long way, it will be well.

I do not apply for indisputable true and I know many weak places in the described technology, but they not impasses - it is simply not up to the end are realized (and it's good:)))
I shall read criticism of other with pleasure too.

PSS: I'm sorry for long message. I shall be in vacation for Monday (from 6 to 20 September) and if someone will want to ask me (or simply to write a message), send it not only in list but duplicate it on my e-mail too, please. I shall be on work today and tomorrow only, but after vacation I shall answer all messages (if I not answer before vacation shall have time).

Published: September 2, 1999

See also
ABA Picture Convert
Paradox ActiveX
Viewer for TNEF-files (winmail.dat)
Paradox Viewer
DBISAM Password Recovery
Paradox to Text converter
Clarion to Text converter
SMExport suite

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