Gets or sets the identifier of a corresponding data member in the data source.
A String that specifies a data member that corresponds to this data item.
To specify the data member identifier, pass it to the constructor of a DataItem descendant when creating its instance.
The data member identifier consists of the data table name and the column name, dot separated. For instance, the Street Address column from the Customers table is identified as "Customers.Street Address".
If a data source has been specified in code, particular data fields can be custom objects. In this instance, the identifier is a chain of property names, like "Customers.City.Population".
Note that aliases specified in the Query Builder are not used in data member identifiers; actual column names are used instead.