We performed the largest and most homogeneous spectroscopic survey of field RR Lyraes (RRLs). We secured ~6300 high-resolution (HR, R~35000) spectra for 143 RRLs (111 fundamental, RRab; 32 first-overtone, RRc). The atmospheric parameters were estimated by using the traditional approach and the iron abundances were measured by using an LTE line analysis. The resulting iron distribution shows a well-defined metal-rich tail approaching solar iron abundance. This suggests that field RRLs experienced a complex chemical enrichment in the early halo formation. We used these data to develop a new calibration of the {Delta}S method. This diagnostic, based on the equivalent widths of Ca^II^K and three Balmer (H{delta},{gamma},{beta}) lines, traces the metallicity of RRLs. For the first time, the new empirical calibration: (I) includes spectra collected over the entire pulsation cycle; (II) includes RRc variables; (III) relies on spectroscopic calibrators covering more than three dex in iron abundance; and (IV) provides independent calibrations based on one/two/three Balmer lines. The new calibrations were applied to a data set of both SEGUE-SDSS and degraded HR spectra totalling 6451 low-resolution (R~2000) spectra for 5001 RRLs (3439 RRab, 1562 RRc). This resulted in an iron distribution with a median {eta}=-1.55{+/-}0.01 and {sigma}=0.51dex, in good agreement with literature values. We also found that RRc are 0.10dex more metal-poor than RRab variables, and have a distribution with a smoother metal-poor tail. This finding supports theoretical prescriptions suggesting a steady decrease in the RRc number when moving from metal-poor to metal-rich stellar environments.