query: Request data from data source

Description

This generic function is the main interface to the data behind the Xdata layer. The first argument is the data object, the second argument is an identifier, usually a string, of the resource requested. For the Twttr class, the query method provides additional optional arguments: user (twitter screenname, default self@user), since_id (character, only tweets with newer id, default NULL), count (numerical, maximum tweets, default NULL) For the Xsparql class the query method relies on code of the authors of the SPARQL package. The resource parameter is interpreted als SPARQL statement, optional parameters are: maxrows (numeric, default=NULL) for limit the rows to fetch, interactive (logical, default FALSE) asks for user input before fetching next rows, typeconv (logical, default TRUE) to convert numbers and dates to R types For the InternalData class, one resource with the same name as the dataset is defined. For the Evs class, the query for the Categories resource (typical expenses by Coicop2 categories) provides additional optional parameters: income (income level, default "(all)"), hhtype (household type, default "(all)"), relative (if TRUE (default), return percentages, otherwise Euro). For the Evs class, the query for the Elasticity resource (Plots expenditures by income group for a given category), an additional parameters categ (character, category for which to plot income elasticity), xlab and ylab (axis titles) are supported. Other parameters will be passed to boxplot. For the GScholar class additional required parameters are "from" and "to", integers (years) that specify the search time span.

Arguments

self

an Xdata object

resource

an identifier of the resource requested. End-user usually provide character, developer use resource and dispatch on the type.

dbconn

an optional SQLiteDataStoreConnection. See details

...

additional parameter

Details

A optional third parameter is dbconn, a SqliteDataStore object. If provided, it tries to locate the resource in the database first and thus complements the scrape method. Depending on the data object, additional parameter can be provided. Inherited classes should override the slot with dbconn="missing". The slot resource should be filled with the help of the resource function. The default (Xdata) implementation with resource as character creates a Resource object of that name and calls itself again with this parameter. Thus dispatching on the resource is possible.