Fix the class of the date attribute on multi networks (due to a bug i…

…n igraph)

As of igraph version 1.4.0, igraph respects the classes of edge and vertex
attributes. However, the function `igraph::disjoint_union` does not respect
their classes and converts `POSIXct` attributes to numeric attributes, for
instance. This is a bug in igraph that is expected to be fixed in later igraph
versions. For now, we temporarily fix this problem on our own by converting the
`date` attribute of the union back to `POSIXct`.
Note: Also other attributes could be affected! However, for convenience reasons,
we only apply our temporary fix to the `date` attribute.

This addresses se-sic#236.

Signed-off-by: Thomas Bock <bockthom@cs.uni-saarland.de>

util-networks.R

@@ -915,6 +915,21 @@ NetworkBuilder = R6::R6Class("NetworkBuilder",
             ## combine the networks:
             ## 1) merge the existing networks
             u = igraph::disjoint_union(authors.net, artifacts.net)
+
+            ## As there is a bug in 'igraph::disjoint_union' in igraph versions 1.4.0 and 1.4.1
+            ## (see https://github.com/igraph/rigraph/issues/761), we need to adjust the type of the date attribute
+            ## of the outcome of 'igraph::disjoint_union'.
+            ## Note: The following temporary fix only considers the 'date' attribute. However, this problem could also
+            ## affect several other attributes, whose classes are not adjusted in our temporary fix.
+            ## The following code block should be redundant as soon as igraph has fixed their bug.
+            u.actual.edge.attribute.date = igraph::get.edge.attribute(u, "date")
+            if (is.list(u.actual.edge.attribute.date)){
+                u.expected.edge.attribute.date = lapply(u.actual.edge.attribute.date, get.date.from.unix.timestamp)
+            } else {
+                u.expected.edge.attribute.date = get.date.from.unix.timestamp(u.actual.edge.attribute.date)
+            }
+            u = igraph::set.edge.attribute(u, "date", value = u.expected.edge.attribute.date)
+
             ## 2) add the bipartite edges
             u = add.edges.for.bipartite.relation(u, authors.to.artifacts, private$network.conf)