Dynamic Database Switching: Difference between revisions
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*NatHols - National Holidays | *NatHols - National Holidays | ||
'''Customer Tables''' (exist many times) | '''Customer Tables''' (exist many times in different databases) | ||
*Customers | *Customers | ||
*Transactions | *Transactions | ||
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===The Database Switching Object=== | ===The Database Switching Object=== | ||
<source lang="vdf"> | <source lang="vdf"> | ||
Use MerTech.inc | Use MerTech.inc | ||
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===DataDictionary Sub-Class=== | ===DataDictionary Sub-Class=== | ||
We then need to define which tables will be "customer" tables - that is the ones which will exist as different versions in different databases. To do this it is most convenient to have a subclass of the Data Access DataDictionary class, eith just in the workspace, or in a library workspace if it is to be used in more than one project (placed in the AppSrc directory it either case). In this we will: | We then need to define which tables will be "customer" tables - that is the ones which will exist as different versions in different databases. To do this it is most convenient to have a subclass of the Data Access DataDictionary class, eith just in the workspace, or in a [[library workspace]] if it is to be used in more than one project (placed in the AppSrc directory it either case). In this we will: | ||
* Create a property in Construct_Object | * Create a property in Construct_Object | ||
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<source lang="vdf"> | <source lang="vdf"> | ||
Use DataDict.pkg | Use DataDict.pkg // | ||
// We want to should ensure that the global handle is defined, so that this | // We want to should ensure that the global handle is defined, so that this | ||
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Register_Procedure AddCustomerTable Integer iTab | Register_Procedure AddCustomerTable Integer iTab | ||
// Our sub-class of the standard DataDictionary class, on which we will base all | |||
// our table data disctionaries: | |||
Class cMyDataDictionary is a DataDictionary | Class cMyDataDictionary is a DataDictionary | ||
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Forward Send End_Construct_Object | Forward Send End_Construct_Object | ||
// IF this is a customer table AND we have the switcher object: | // IF this is a customer table AND we have the switcher object, call | ||
// the procedure passing it the file number: | |||
If (pbCustomerTable(Self) and (ghoDbSwitch <> 0)) ; | If (pbCustomerTable(Self) and (ghoDbSwitch <> 0)) ; | ||
Send AddCustomerTable of ghoDbSwitch (Main_File(Self)) | Send AddCustomerTable of ghoDbSwitch (Main_File(Self)) | ||
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End_Class // cMyDataDictionary | End_Class // cMyDataDictionary | ||
</source> | </source> | ||
In the [[Visual DataFlex Studio]] you should then set this sub-class to be the super-class for all of your data dictionaries: Tools -> Configure Workspace -> Class Preferences tab -> DataDictionary and put cMyDatadictionary (or whatever you are calling yours) and cMyDataDictionary.pkg in the two columns. | |||
===Data Disctionaries=== | |||
In the data dictionary class file for each of your tables, you should ensure that these are based on your data dictionary sub-class, then, if the table to be marked as a "customer" table, set that property in the Construct_Object procedure: | |||
<source lang="vdf"> | |||
Use cMyDataDictionary.pkg // DataDictionary Class Definition | |||
Open Customer | |||
Class Customer_DataDictionary is a cMyDataDictionary | |||
Procedure Construct_Object | |||
Forward Send Construct_Object | |||
Set pbCustomerTable to True | |||
End_Procedure // Construct_Object | |||
End_Class // Customer_DataDictionary | |||
</source> | |||
===Without Data Dictionaries=== | |||
If you need to have the database switching capability for tables you are opening directly, instead of using data dictionaries for them, you can just use the same call that was place in the data dictionary sub-class, but using the table's File_Number rather than the Main_File property: | |||
<source lang="vdf"> | |||
If (ghoDbSwitch <> 0) Send AddCustomerTable of ghoDbSwitch Customer.File_Number | |||
</source> | |||
===Swiching Database=== | |||
Finally, having set up all the required infrastructure, actually invoking the database change becomes very simple. Each code module which uses it should implement the conditional declaration and initialisation of the global handle and register the SwitchDB procedure to ensure it will not cause compile-time or run-time errors if the Database Switcher is not present: | |||
<source lang="vdf"> | |||
#IFDEF ghoDbSwitch | |||
#ELSE | |||
Global_Variable Handle ghoDbSwitch | |||
Move 0 to ghoDbSwitch // Initalise to zero | |||
#ENDIF | |||
Register_Procedure SwitchDB String sDatabase | |||
</source> | |||
Then switching database becomes a simple matter of invoking that procedure: | |||
<source lang="vdf"> | |||
If ghoDbSwitch Send SwitchDB sDatabase | |||
</source> | |||
This might be invoked in the login module of some applications, where the database to use is determined by the user's identity, or in the Session Manager module of an [[Ajax]] [[Web Application]], or simply in the change of a visual control (such as a Combo Form) in an application in which the user can choose between many databases to work on. | |||