  We present some homological properties of a relation $\beta$ on ordered
groupoids that generalises the minimum group congruence for inverse semigroups.
When $\beta$ is a transitive relation on an ordered groupoid $G$, the quotient
$G / \beta$ is again an ordered groupoid, and construct a pair of adjoint
functors between the module categories of $G$ and of $G / \beta$. As a
consequence, we show that the homology of $G$ is completely determined by that
of $G / \beta$, generalising a result of Loganathan for inverse semigroups.
