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The foreign key constraint can fail when attempting to save a new record in Rails if the value of the foreign key does not exist in the corresponding primary key column of the associated table. This means that the record being saved is trying to reference a record that does not exist in the associated table. To resolve this, the foreign key value must match an existing primary key value, or a new record must be created in the associated table to match the foreign key value. Additionally, other factors such as database constraints and validations can also contribute to the failure of foreign key constraints.