Preparing Data in a Datalayer

Data from a database is usually retrieved in Logi applications using a datalayer element. The File Column child element accesses data in a BLOB or CLOB column by manipulating the columns in a datalayer. This is similar to the Calculated Column element, which can be used to add additional columns to a datalayer. The File Column element performs these operations:

• Reads the BLOB or CLOB data from the datalayer and saves it to a temporary file, then deletes it from the datalayer.
• Adds two columns to the datalayer and inserts values into them for the temporary file's path and filename.

The following example illustrates how the File Column element is used:

1. As shown above, a File Column element is added beneath a datalayer; multiple File Column elements can be used as necessary.
2. Its required Data Column attribute specifies the name of the column in the datalayer that contains the BLOB or CLOB data.
3. Its required Filename attribute specifies the fully-qualified path and filename for the temporary file that will hold the BLOB or CLOB data. The example above uses one @Function token to get the Logi application's base folder path and another to create a unique identifier for the actual file name. The complete value might look like this:
   
   @Function.AppPhysicalPath~\rdDownload\@Function.GUID~.png
   
   Use of the rdDownload folder as the location for temporary files within your application folder ensures that they'll be "cleaned-up" automatically soon after each user's session ends. Don't forget to provide a file extension appropriate to the data type, such as .png, .jpg, .doc, .xml, etc.
   
   If a file of the same name already exists in the specified location, it will be overwritten without warning.

4. Its required ID attribute specifies a unique identifier for the File Column element.
5. Its optional Data Type attribute specifies the type, Text or Binary, of the BLOB or CLOB data. Binary values should be Base64-encoded (this is typical).

The configuration of the File Column element thus far has satisfied the first of the two operations mentioned earlier, saving the data to a temporary file and then automatically removing it from the datalayer. The final two optional File Column element attributes are discussed in Working with the Retrieved Data.