#### bb file ####

DEPENDS = "postgresql"
RDEPENDS_${PN} = "bash"

[...]

inherit classfile

#### classfile ####

DEPENDS += " \
  ruby-native \
  postgresql \
"
RDEPENDS_${PN} += " \
  ruby \
"

[...]

classfile_do_install() {
  gem install pg -v '0.19.0' -- --with-pg-lib="${STAGING_DIR_NATIVE}/usr/lib"
}