src/Normalizer/EntityNormalizer.php | 39 +++++++++++++++++++++++++++----------
 1 file changed, 29 insertions(+), 10 deletions(-)

diff --git a/src/Normalizer/EntityNormalizer.php b/src/Normalizer/EntityNormalizer.php
index 0f3e99d..5b3167f 100644
--- a/src/Normalizer/EntityNormalizer.php
+++ b/src/Normalizer/EntityNormalizer.php
@@ -121,21 +121,37 @@ class EntityNormalizer extends NormalizerBase implements DenormalizerInterface {
       $entity->bundle()
     );
     $resource_type_name = $resource_type->getTypeName();
+    // Get the bundle ID of the requested resource. This is used to determine if
+    // this is a bundle level resource or an entity level resource.
+    $bundle = $resource_type->getBundle();
+    if (!empty($context['sparse_fieldset'][$resource_type_name])) {
+      $field_names = $context['sparse_fieldset'][$resource_type_name];
+    }
+    else {
+      $field_names = $this->getFieldNames($entity, $bundle, $resource_type);
+    }
 
     $cached_normalization_parts = $this->cacheGet($resource_type, $entity);
     if ($cached_normalization_parts !== FALSE) {
       $normalization_parts = $cached_normalization_parts;
+      // Check if some of the currently requested sparse fieldset has not yet
+      // been normalized in the cache item. If there are any missing fields,
+      // normalize those too, and update the cache.
+      $missing = array_diff($field_names, array_keys($normalization_parts['fields']));
+      if ($missing) {
+        $missing_normalization_parts = $this->getPartialNormalization($missing, $entity, $resource_type, $format, $context);
+        $normalization_parts['fields'] += $missing_normalization_parts['fields'];
+        $this->cacheSet($resource_type, $entity, $normalization_parts);
+      }
     }
     else {
-      $normalization_parts = $this->getPartialNormalization($entity, $resource_type, $format, $context);
+      $normalization_parts = $this->getPartialNormalization($field_names, $entity, $resource_type, $format, $context);
       $this->cacheSet($resource_type, $entity, $normalization_parts);
     }
 
-    // If a sparse fieldset is active during this normalization, keep only the
-    // requested fields.
-    $field_normalizations = (!empty($context['sparse_fieldset'][$resource_type_name]))
-      ? array_intersect_key($normalization_parts['fields'], array_flip($context['sparse_fieldset'][$resource_type_name]))
-      : $normalization_parts['fields'];
+    // Keep only the requested fields (the cached normalization gradually grows
+    // to the complete set of fields).
+    $field_normalizations = array_intersect_key($normalization_parts['fields'], array_flip($field_names));
 
     $entity_normalizaton = $normalization_parts['base'];
     $relationship_field_names = array_keys($resource_type->getRelatableResourceTypes());
@@ -149,6 +165,8 @@ class EntityNormalizer extends NormalizerBase implements DenormalizerInterface {
   /**
    * Normalizes an entity into a partial normalization.
    *
+   * @param string[] $field_names
+   *   The field names to normalize (the sparse fieldset, if any).
    * @param \Drupal\Core\Entity\EntityInterface $entity
    *   The entity to partially normalize.
    * @param \Drupal\jsonapi\ResourceType\ResourceType $resource_type
@@ -162,15 +180,16 @@ class EntityNormalizer extends NormalizerBase implements DenormalizerInterface {
    *   An array with two key-value pairs:
    *   - 'base': array, the base normalization of the entity, that does not
    *             depend on which sparse fieldset was requested.
-   *   - 'fields': CacheableNormalization for every field on this entity, of
-   *               which a subset will need to be selected depending on the
-   *               requested sparse fieldset.
+   *   - 'fields': CacheableNormalization for all requested fields.
    *
    * @see ::normalize()
    */
-  private function getPartialNormalization(EntityInterface $entity, ResourceType $resource_type, $format = NULL, array $context = []) {
+  private function getPartialNormalization(array $field_names, EntityInterface $entity, ResourceType $resource_type, $format = NULL, array $context = []) {
     $normalizer_values = [];
     foreach ($this->getFields($entity, $resource_type->getBundle(), $resource_type) as $field_name => $field) {
+      if (!in_array($field_name, $field_names, TRUE)) {
+        continue;
+      }
       $normalizer_values[$field_name] = $this->serializeField($field, $context, $format);
     }
     // Create the array of normalized fields, starting with the URI.