rent recieved in advance journal entry is

rent A/c... Dr

to rent in advance A/c

but why we cant debit cash A/c because cash is coming..

Basically the entry for rent received in advance is 

Cash/Bank A/c Dr.  xxx

 To rent received in advance  xxx

 (Being Rent received in advance)

The entry provided by you is discussed in chapter Rectification's of Errors. This entry is used at year end when we charge rent from rent received in advance account.