The origin and cosmic evolution of the gas metallicity and stellar metallicity vs galaxy mass relations are a matter of hot debate. Usually, these relations are studied separately, and there is no comprehensive model that can explain both simultaneously. As the stellar metallicity carries information about the early epochs of chemical enrichment in galaxies, while gas metallicity reflects more recent evolutionary processes, the simultaneous study of both will set the stringent constraints to galaxy formation and evolution across cosmic time. Here we show a unified model aiming at reproducing both relations simultaneously. As input data we use integrated spectra from SDSS, with carefully derived stellar and gas metallicities.