A "direct relationship" refers to a connection or link between two things where changes in one thing cause corresponding changes in the other thing. Full definition
There's a kind of direct relationship with the body in the image. (frieze.com)
The two practices often inform each other and recently there has been a more direct relationship between them. (painters-table.com)
We found links between all elements of our theoretical framework, with some having a more direct relationship with student learning. (wallacefoundation.org)