oneMemberRelationCheck.md

One Member Relation Check

Description

This check identifies relations that only contain one member. In OSM, a Relation is a multi-purpose data structure that documents a relationship between two or more data elements (nodes, ways, and/or other relations).

Live Example

The following examples illustrate two cases where a Relation contains only one member.

1. This relation id:4646212 has only one member.
2. This relation id:7691824 has only one member.

Code Review

In Atlas, OSM elements are represented as Edges, Points, Lines, Nodes & Relations; in our case, we’re working with Relations.

Our first goal is to validate the incoming Atlas Object. We use some preliminary filtering to target Relation objects. Therefore, we use:

• Must be a Relation
• Must not have been flagged before

    @Override
        public boolean validCheckForObject(final AtlasObject object)
        {
            return object instanceof Relation;
        }

After the preliminary filtering of features, we get the members of that relation. If the number of members in the relation is 1, then we flag the relation as problematic. One thing to note is that multi-polygon relations with one member are flagged with a more specific instruction than the regular one member relation.

      @Override
          protected Optional<CheckFlag> flag(final AtlasObject object)
          {
              final Relation relation = (Relation) object;
              final RelationMemberList members = relation.members();
      
              // If the number of members in the relation is 1
              if (members.size() == 1)
              {
                  // If the relation is a multi-polygon,
                  if (relation.isMultiPolygon()) {
                      return Optional.of(createFlag(
                              relation.members().stream().map(RelationMember::getEntity)
                                      .collect(Collectors.toSet()),
                              this.getLocalizedInstruction(1, relation.getOsmIdentifier())));
                  }
                  return Optional.of(createFlag(
                          relation.members().stream().map(RelationMember::getEntity)
                                  .collect(Collectors.toSet()),
                          this.getLocalizedInstruction(0, relation.getOsmIdentifier())));
              }
              return Optional.empty();
          }

To learn more about the code, please look at the comments in the source code for the check. OneMemberRelationCheck.java