1. A method comprising: determining, by a computer system, a plurality of nodes, wherein each node of the plurality of nodes comprises a corresponding set of criteria; determining, by the computer system, a plurality of paths among the plurality of nodes, wherein the plurality of paths define a relationship among the plurality of nodes; determining, by the computer system, a transitional operator associated with a path in the plurality of paths connecting a first node and a second node in the plurality of nodes, wherein the transitional operator defines an additional criterion for the first node and the second node; and retrieving, by the computer system, a plurality of content items that include characteristics that are determined to match the corresponding set of criteria for at least one node in the plurality of nodes, wherein a first content item in the plurality of content items for the first node and a second content item for the second node are retrieved based on the first content item having a first characteristic and the second content item having a second characteristic that satisfy the additional criterion of the transitional operator.