[ML] Improve error when no available field exists for rule scope (#32550)

Closes #32542

Author: dimitris-athanasiou

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/job/config/RuleScope.java

@@ -84,6 +84,10 @@ public boolean isEmpty() {
     public void validate(Set<String> validKeys) {
         Optional<String> invalidKey = scope.keySet().stream().filter(k -> !validKeys.contains(k)).findFirst();
         if (invalidKey.isPresent()) {
+            if (validKeys.isEmpty()) {
+                throw ExceptionsHelper.badRequestException(Messages.getMessage(Messages.JOB_CONFIG_DETECTION_RULE_SCOPE_NO_AVAILABLE_FIELDS,
+                        invalidKey.get()));
+            }
             throw ExceptionsHelper.badRequestException(Messages.getMessage(Messages.JOB_CONFIG_DETECTION_RULE_SCOPE_HAS_INVALID_FIELD,
                     invalidKey.get(), validKeys));
         }

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/job/messages/Messages.java

@@ -95,6 +95,8 @@ public final class Messages {
             "Invalid detector rule: function {0} only supports conditions that apply to time";
     public static final String JOB_CONFIG_DETECTION_RULE_REQUIRES_SCOPE_OR_CONDITION =
             "Invalid detector rule: at least scope or a condition is required";
+    public static final String JOB_CONFIG_DETECTION_RULE_SCOPE_NO_AVAILABLE_FIELDS =
+            "Invalid detector rule: scope field ''{0}'' is invalid; detector has no available fields for scoping";
     public static final String JOB_CONFIG_DETECTION_RULE_SCOPE_HAS_INVALID_FIELD =
             "Invalid detector rule: scope field ''{0}'' is invalid; select from {1}";
     public static final String JOB_CONFIG_FIELDNAME_INCOMPATIBLE_FUNCTION = "field_name cannot be used with function ''{0}''";

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/job/config/RuleScopeTests.java

@@ -10,6 +10,8 @@
 import org.elasticsearch.common.util.set.Sets;
 import org.elasticsearch.test.AbstractWireSerializingTestCase;
 
+import java.util.Collections;
+
 import static org.hamcrest.Matchers.contains;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
@@ -53,6 +55,17 @@ public void testValidate_GivenMultipleValidFields() {
         scope.validate(Sets.newHashSet("foo", "bar", "foobar"));
     }
 
+    public void testValidate_GivenNoAvailableFieldsForScope() {
+        RuleScope scope = RuleScope.builder()
+                .include("foo", "filter1")
+                .build();
+        assertThat(scope.isEmpty(), is(false));
+
+        ElasticsearchStatusException e = expectThrows(ElasticsearchStatusException.class, () -> scope.validate(Collections.emptySet()));
+        assertThat(e.getMessage(), equalTo("Invalid detector rule: scope field 'foo' is invalid; " +
+                "detector has no available fields for scoping"));
+    }
+
     public void testValidate_GivenMultipleFieldsIncludingInvalid() {
         RuleScope scope = RuleScope.builder()
                 .include("foo", "filter1")