In Drupal 8, we have a BlogPost content type which has a field for 'Blog Category'. The blog categories are predefined in a taxonomy list and they are linked to BlogPost 'Blog Category' field via BlogCategory entity reference. If the user enters a letter in the 'Blog Category' field, Drupal will try to auto-populate the input field by displaying a dynamic drop down list containing BlogCategories. However, we often have situations where the user ignores the drop-down suggestion list, and enters a custom category name. i.e. Taxonomy category is named 'Books and CDs', while the user would type 'Books&CDs'. What we're trying to do is force the user to select an existing blog category from predefined BlogCategory taxonomy list, and prevent them from entering a new, custom named category.
