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-rw-r--r--guides/source/association_basics.md6
1 files changed, 6 insertions, 0 deletions
diff --git a/guides/source/association_basics.md b/guides/source/association_basics.md
index 9b80a65a44..e133e71d42 100644
--- a/guides/source/association_basics.md
+++ b/guides/source/association_basics.md
@@ -1137,6 +1137,12 @@ Controls what happens to the associated object when its owner is destroyed:
* `:restrict_with_exception` causes an exception to be raised if there is an associated record
* `:restrict_with_error` causes an error to be added to the owner if there is an associated object
+It's necessary not to set or leave `:nullify` option for those associations
+that have `NOT NULL` database constraints. If you don't set `dependent` to
+destroy such associations you won't be able to change the associated object
+because initial associated object foreign key will be set to unallowed `NULL`
+value.
+
##### `:foreign_key`
By convention, Rails assumes that the column used to hold the foreign key on the other model is the name of this model with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly: