close #511810: bad rql generated when looking for vocabulary for a relation on an entity which doesn't exist (yet)
technical details:
* add a graph of linked variables in the RRQLExpression instance
* modify rql rewriter to remove relations where an unexistant
variable and or a variable linked to an unexistant variable is used.
* activate this feature in Entity.unrelated_rql if the entity doesn't exists yet
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html