Eric Smith
--- a/src/Feeds/Processor/EntityProcessorBase.php
+++ b/src/Feeds/Processor/EntityProcessorBase.php
+    $skipped_mapping_deltas = [];
+    $incomplete_mapping_deltas = [];
+
+    // Mappers add to existing fields rather than replacing them unless partial
+    // updates are allowed. Hence we need to clear target elements of each item
+    // before mapping in case we are mapping on a prepopulated item such as an
+    // existing node.
    foreach ($mappings as $delta => $mapping) {
      if ($mapping['target'] == 'feeds_item') {
        // Skip feeds item as this field gets default values before mapping.
        continue;
      }

+      if ($missing_source !== static::MISSING_SOURCE_REQUIRE_NONE) {
+        $source_is_missing_from_item = function ($source) use ($item) {
+          return $item->get($source) === NULL && !array_key_exists($source, $item->toArray());
--- a/src/Feeds/Processor/EntityProcessorBase.php
+++ b/src/Feeds/Processor/EntityProcessorBase.php
        }

        $value = $item->get($source);
-        if (!is_array($value)) {
+        if ($value === NULL && !array_key_exists($source, $item->toArray()) && isset($target_values[$delta])) {
--- a/src/Feeds/Processor/Form/DefaultEntityProcessorForm.php
+++ b/src/Feeds/Processor/Form/DefaultEntityProcessorForm.php
      ],
    ];

+    $form['advanced']['missing_source'] = [
+      '#type' => 'radios',
+      '#title' => $this->t('When a mapped source is missing from the feed'),
+      '#options' => [
+        ProcessorInterface::MISSING_SOURCE_REQUIRE_NONE => $this->t('Update all mapped fields, treat the missing sources as NULL'),
+        ProcessorInterface::MISSING_SOURCE_REQUIRE_ALL => $this->t('Update fields where the field mapping has all sources provided'),
+        ProcessorInterface::MISSING_SOURCE_REQUIRE_ANY => $this->t('Update fields where the field mapping has any source provided'),
+      ],
+      '#description' => $this->t("When a field mapping is composed of multiple sources, e.g. for a <em>Text (formatted, long, with summary)</em> field where both the <em>Text</em> and <em>Summary</em> targets are mapped, 'Update fields where the field mapping has all sources provided' will not update the field if either of the source values are missing. Use 'Update fields where the field mapping has any source provided' to merge the existing field column values with supplied field columns from the source.<br><strong>Warning: This may have unexpected consequences for multi value fields.</strong>"),
--- a/src/Feeds/Processor/EntityProcessorBase.php
+++ b/src/Feeds/Processor/EntityProcessorBase.php
+        if ($number_of_missing_sources > 0) {
+          if ($missing_source === static::MISSING_SOURCE_REQUIRE_ANY && $number_of_missing_sources !== count($mapping['map'])) {
+            // Track that this field mapping is incomplete so that we can merge
+            // the existing field columns with provided columns.
+            $incomplete_mapping_deltas[] = $delta;
+          }
+          else {
+            // Track that this field mapping should be skipped completely.
+            $skipped_mapping_deltas[] = $delta;
+          }
+
+          // Do not clear the target as it will either be skipped or merged.
+          continue;
+        }
+      }
+
--- a/src/Feeds/Processor/EntityProcessorBase.php
+++ b/src/Feeds/Processor/EntityProcessorBase.php
+  protected function groupMappingsByAction(array $mappings, ItemInterface $item): array {
+    $skip_missing_source = $this->configuration['skip_missing_source'];
+
+    $grouped = [
+      self::FIELD_MAPPING_IMPORT => [],
+      self::FIELD_MAPPING_IMPORT_KEEP_MISSING_PROPERTIES => [],
+      self::FIELD_MAPPING_SKIP => [],
+    ];
+
+    // Import everything if we do not require source values to be present.
+    if (!$skip_missing_source) {
+      $grouped[self::FIELD_MAPPING_IMPORT] = $mappings;
+      return $grouped;
+    }
+
+    // @todo: Refactor to `has` method on item.
--- a/src/Feeds/Processor/Form/DefaultEntityProcessorForm.php
+++ b/src/Feeds/Processor/Form/DefaultEntityProcessorForm.php
      ],
    ];

+    $form['advanced']['skip_missing_source'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Only process mappings found in the source'),
+      '#description' => $this->t('This will avoid wiping existing target data if a source is missing for a mapping. If a mapping is composed of multiple properties this will update only the properties that have a source provided. E.g. for a <em>Text (formatted, long, with summary)</em> field where both the <em>Text</em> and <em>Summary</em> properties are mapped but only the <em>Summary</em> source is provided then the existing value for <em>Text</em> target will be retained.'),
+      '#default_value' => $this->plugin->getConfiguration('skip_missing'),
--- a/src/Feeds/Processor/Form/DefaultEntityProcessorForm.php
+++ b/src/Feeds/Processor/Form/DefaultEntityProcessorForm.php
      ],
    ];

+    $form['advanced']['skip_missing_source'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Only process mappings found in the source'),
+      '#description' => $this->t('This will avoid wiping existing target data if a source is missing for a mapping. If a mapping is composed of multiple properties this will update only the properties that have a source provided. E.g. for a <em>Text (formatted, long, with summary)</em> field where both the <em>Text</em> and <em>Summary</em> properties are mapped but only the <em>Summary</em> source is provided then the existing value for <em>Text</em> target will be retained.'),
+      '#default_value' => $this->plugin->getConfiguration('skip_missing'),
+      '#parents' => ['processor_configuration', 'skip_missing'],
+      '#states' => [
+        'visible' => [
+          'input[name="processor_configuration[skip_missing_source]"]' => [
--- a/src/Feeds/Processor/EntityProcessorBase.php
+++ b/src/Feeds/Processor/EntityProcessorBase.php
+
+    $grouped = [
+      self::FIELD_MAPPING_IMPORT => [],
+      self::FIELD_MAPPING_IMPORT_KEEP_MISSING_PROPERTIES => [],
+      self::FIELD_MAPPING_SKIP => [],
+    ];
+
+    // Import everything if we do not require source values to be present.
+    if (!$skip_missing_source) {
+      $grouped[self::FIELD_MAPPING_IMPORT] = $mappings;
+      return $grouped;
+    }
+
+    // @todo: Refactor to `has` method on item.
+    $source_is_missing_from_item = function ($source) use ($item) {
+      return $item->get($source) === NULL && !array_key_exists($source, $item->toArray());