add a string example for self-refenretial many-to-many
add a string example for self-refenretial many-to-many

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