BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Data Access Layer and Data Modules          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message Mark Reichert Guest Posted: Tue Jan 16, 2007 5:00 am    Post subject: Data Access Layer and Data Modules I want to make a Data Module deal only with direct database access and put a Data Access Object between it and the applicable form or frame. From a previous thread I conclude that this isn't an original idea. What I can't quite grasp is some of the mechanics. Do you encapsulate the Start of a Transaction and Open/ExecuteQuery in one Datamodule method and the Close, Commit in another? In between, do you populate Data Module fields immediately and have the Data Object collect it afterwards? This would have the two advantages of keeping the transaction time short and keep the field names just in the Data Module. I also suppose that if you are getting data from one table for certain IDs and from two other tables for other IDs, the Data Module should be the one to deal with that detail as well, rather than the Data Access Object knowing there are different tables involved. Are there any books or good examples with have Data Access Objects intereacting with Data Modules? Back to top Mark Reichert Guest Posted: Tue Jan 16, 2007 9:34 pm    Post subject: Re: Data Access Layer and Data Modules "Mark Reichert" <mark (AT) messagelink (DOT) com> wrote in message news:45ac06a3$1 (AT) newsgroups (DOT) borland.com... Quote: Do you encapsulate the Start of a Transaction and Open/ExecuteQuery in one Datamodule method and the Close, Commit in another? In between, do you populate Data Module fields immediately and have the Data Object collect it afterwards? This would have the two advantages of keeping the transaction time short and keep the field names just in the Data Module. I also suppose that if you are getting data from one table for certain IDs and from two other tables for other IDs, the Data Module should be the one to deal with that detail as well, rather than the Data Access Object knowing there are different tables involved. While I'm waiting for other answers, I've decided to start experimenting with something I found in a thread here in 2005. Someone (who I would give attribution to if I sure he wanted to be mentioned) wrote that before he started using a full fledged OPF, he use the intermediate step of something like this: DataModule.SaveCustomer(ACustomer: TCustomer). In other words, letting the Data Module as well as the Forms know about the business class, but only so it can load up the class with info from the databases or save data from the classes to the database. I know classes well enough to be comfortable with passing the data around through this method, though there's still the issue of when you load and when you save. I still haven't managed to get a frame properly initialized on startup because I can't find the exact spot given Delphi's quirky order of activation. http://groups.google.com/group/borland.public.delphi.ide/msg/2225f83d74cf6920?hl=en& Back to top Mark Reichert Guest Posted: Thu Jan 18, 2007 3:03 am    Post subject: Re: Data Access Layer and Data Modules "Mark Reichert" <mark (AT) messagelink (DOT) com> wrote in message news:45acefcc$1 (AT) newsgroups (DOT) borland.com... Quote: "Mark Reichert" <mark (AT) messagelink (DOT) com> wrote in message news:45ac06a3$1 (AT) newsgroups (DOT) borland.com... While I'm waiting for other answers, I've decided to start experimenting with something I found in a thread here in 2005. Someone (who I would give attribution to if I sure he wanted to be mentioned) wrote that before he started using a full fledged OPF, he use the intermediate step of something like this: DataModule.SaveCustomer(ACustomer: TCustomer). In other words, letting the Data Module as well as the Forms know about the business class, but only so it can load up the class with info from the databases or save data from the classes to the database. I know classes well enough to be comfortable with passing the data around through this method, though there's still the issue of when you load and when you save. Or there is TCustomer.SaveToDatabase(ADataModule: TDataModule). I have no idea if that would work any better, but it would keep the Data Module from knowing anything about the business class, even for the purpose of loading or saving. Back to top Lars Frische Guest Posted: Thu Jan 18, 2007 9:12 am    Post subject: Re: Data Access Layer and Data Modules Mark Reichert wrote: Quote: Or there is TCustomer.SaveToDatabase(ADataModule: TDataModule). The problem with this approach is that you cannot easily exchange the persistence layer (flat file/xml vs. database X vs. database Y). If you are 100% percent sure that this won't likely change then this is probably a viable (if not really nice) way. Cheers Lars -- Lars Frische Software Developer Test Maschinen Technik GmbH Germany === http://www.eddyMax.com http://www.kontrolltechnik.com Back to top Mark Reichert Guest Posted: Thu Jan 18, 2007 9:17 pm    Post subject: Re: Data Access Layer and Data Modules "Lars Frische" <nospam (AT) eddymax (DOT) com> wrote in message news:45af22ad$1 (AT) newsgroups (DOT) borland.com... Quote: Mark Reichert wrote: Or there is TCustomer.SaveToDatabase(ADataModule: TDataModule). The problem with this approach is that you cannot easily exchange the persistence layer (flat file/xml vs. database X vs. database Y). If you are 100% percent sure that this won't likely change then this is probably a viable (if not really nice) way. Only if you run into something you can't do inside a TDataModule. I was assuming that if you passed a TDataModule variable as a parameter, the Business Class could see the TDataModule properties you've add, properties that correspond to Business Class members. The Business Class wouldn't have to know much if anything about where or how the TDataModule loads from or saves to storage. Please tell me if I'm incorrect and where or what I should be reading to learn about real world practical solutions. Back to top Franz-Leo Chomse Guest Posted: Thu Jan 18, 2007 11:14 pm    Post subject: Re: Data Access Layer and Data Modules Quote: Only if you run into something you can't do inside a TDataModule. I was assuming that if you passed a TDataModule variable as a parameter, the Business Class could see the TDataModule properties you've add, properties that correspond to Business Class members. The Business Class wouldn't have to know much if anything about where or how the TDataModule loads from or saves to storage. Only if the business class uses a sort of dynamic binding like access properties via RTTI. If you use static binding you only see the original TDataModule properties. Regards from Germany Franz-Leo Back to top Mark Reichert Guest Posted: Thu Jan 18, 2007 11:46 pm    Post subject: Re: Data Access Layer and Data Modules "Franz-Leo Chomse" <franz-leo.chomse (AT) samac (DOT) de> wrote in message news:odavq2dr7icajd008ih6mlfdet7s2j9n25 (AT) 4ax (DOT) com... Quote: Only if you run into something you can't do inside a TDataModule. I was assuming that if you passed a TDataModule variable as a parameter, the Business Class could see the TDataModule properties you've add, properties that correspond to Business Class members. The Business Class wouldn't have to know much if anything about where or how the TDataModule loads from or saves to storage. Only if the business class uses a sort of dynamic binding like access properties via RTTI. If you use static binding you only see the original TDataModule properties. I'll have to pass the paramater as the actual TDataModule descendant class type then. Perhaps I should make a generic TDataModule descendant class with the proper property declarations only, make that the class I pass, and descend any specific instance with the actual dataset work from that. I'll do a search for example of anybody doing that. I know I'm behind the times on this (if ahead here at this company), but I haven't found any solid examples of using business classes that still allow me to use Data Modules, hence my meandering posts here. 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