ESQL: Fix some more usages of field attribute names in LOOKUP JOIN

x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/expression/FieldAttribute.java

@@ -34,8 +34,8 @@
  *
  * To adequately represent e.g. union types, the name of the attribute can be altered because we may have multiple synthetic field
  * attributes that really belong to the same underlying field. For instance, if a multi-typed field is used both as {@code field::string}
- * and {@code field::ip}, we'll generate 2 field attributes called {@code $$field$converted_to$string} and {@code $$field$converted_to$ip}
- * but still referring to the same underlying field.
+ * and {@code field::ip}, we'll generate 2 field attributes called {@code $$field$converted_to$keyword} and {@code $$field$converted_to$ip}
+ * which still refer to the same underlying index field.
  */
 public class FieldAttribute extends TypedAttribute {
 
@@ -163,6 +163,15 @@ public FieldName fieldName() {
         return lazyFieldName;
     }
 
+    /**
+     * The name of the attribute. Can deviate from the field name e.g. in case of union types. For the physical field name, use
+     * {@link FieldAttribute#fieldName()}.
+     */
+    @Override
+    public String name() {
+        return super.name();
+    }
+
     public EsField.Exact getExactInfo() {
         return field.getExactInfo();
     }
@@ -176,7 +185,7 @@ public FieldAttribute exactAttribute() {
     }
 
     private FieldAttribute innerField(EsField type) {
-        return new FieldAttribute(source(), name(), name() + "." + type.getName(), type, nullable(), id(), synthetic());
+        return new FieldAttribute(source(), fieldName().string, name() + "." + type.getName(), type, nullable(), id(), synthetic());
     }
 
     @Override

x-pack/plugin/esql/qa/testFixtures/src/main/resources/lookup-join.csv-spec

@@ -732,6 +732,28 @@ language.id:integer | language.name:text | language.name.keyword:keyword | langu
 2                   | French             | French                        | FR
 ;
 
+###############################################
+# union type behavior
+###############################################
+
+joinOnMultiTypedMatchFieldCastToInteger
+required_capability: join_lookup_v12
+
+FROM apps, apps_short METADATA _index
+| EVAL language_code = id::integer
+| KEEP _index, language_code
+| WHERE language_code < 3
+| LOOKUP JOIN languages_lookup ON language_code
+| SORT _index ASC, language_code ASC
+;
+
+_index:keyword | language_code:integer | language_name:keyword
+apps           | 1                     | English
+apps           | 2                     | French
+apps_short     | 1                     | English
+apps_short     | 2                     | French
+;
+
 ###############################################
 # Tests with clientips_lookup index
 ###############################################

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/action/LookupFromIndexIT.java

@@ -54,6 +54,7 @@
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.xpack.core.async.AsyncExecutionId;
 import org.elasticsearch.xpack.esql.core.expression.Alias;
+import org.elasticsearch.xpack.esql.core.expression.FieldAttribute;
 import org.elasticsearch.xpack.esql.core.expression.ReferenceAttribute;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
@@ -229,7 +230,7 @@ private void runLookup(DataType keyType, PopulateIndices populateIndices) throws
                 keyType,
                 "lookup",
                 "lookup",
-                "key",
+                new FieldAttribute.FieldName("key"),
                 List.of(new Alias(Source.EMPTY, "l", new ReferenceAttribute(Source.EMPTY, "l", DataType.LONG))),
                 Source.EMPTY
             );