some fk fixes for PG
some fk fixes for PG

hsum/sqlalchemy,graingert/sqlalchemy,sqlalchemy/sqlalchemy,monetate/sqlalchemy,alex/sqlalchemy,sandan/sqlalchemy,itkovian/sqlalchemy,davidjb/sqlalchemy,Akrog/sqlalchemy,j5int/sqlalchemy,alex/sqlalchemy,davidfraser/sqlalchemy,monetate/sqlalchemy,pdufour/sqlalchemy,bootandy/sqlalchemy,pdufour/sqlalchemy,sandan/sqlalchemy,wfxiang08/sqlalchemy,robin900/sqlalchemy,elelianghh/sqlalchemy,ThiefMaster/sqlalchemy,EvaSDK/sqlalchemy,EvaSDK/sqlalchemy,276361270/sqlalchemy,halfcrazy/sqlalchemy,epa/sqlalchemy,robin900/sqlalchemy,wfxiang08/sqlalchemy,brianv0/sqlalchemy,WinterNis/sqlalchemy,WinterNis/sqlalchemy,olemis/sqlalchemy,olemis/sqlalchemy,inspirehep/sqlalchemy,graingert/sqlalchemy,bdupharm/sqlalchemy,inspirehep/sqlalchemy,halfcrazy/sqlalchemy,davidjb/sqlalchemy,alex/sqlalchemy,zzzeek/sqlalchemy,elelianghh/sqlalchemy,epa/sqlalchemy,dstufft/sqlalchemy,276361270/sqlalchemy,bdupharm/sqlalchemy,wujuguang/sqlalchemy,wujuguang/sqlalchemy,Cito/sqlalchemy,brianv0/sqlalchemy,hsum/sqlalchemy,dstufft/sqlalchemy,Cito/sqlalchemy,Akrog/sqlalchemy,davidfraser/sqlalchemy,bootandy/sqlalchemy,j5int/sqlalchemy,ThiefMaster/sqlalchemy,Cito/sqlalchemy,itkovian/sqlalchemy