[1.8.x] Refs #2495 -- Documented that MySQL cannot have TextField(unique=True).

Backport of e8cbc2b322d873a6fe39faca18d340ce93035087 from master

1 files changed, 8 insertions, 0 deletions

diff --git a/docs/ref/databases.txt b/docs/ref/databases.txt
index 1b601234bf..227b5b7921 100644
--- a/docs/ref/databases.txt
+++ b/docs/ref/databases.txt
@@ -500,6 +500,14 @@ for the field. This affects :class:`~django.db.models.CharField`,
 :class:`~django.db.models.SlugField` and
 :class:`~django.db.models.CommaSeparatedIntegerField`.
 
+``TextField`` limitations
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+MySQL can index only the first N chars of a ``BLOB`` or ``TEXT`` column. Since
+``TextField`` doesn't have a defined length, you can't mark it as
+``unique=True``. MySQL will report: "BLOB/TEXT column '<db_column>' used in key
+specification without a key length".
+
 .. _mysql-fractional-seconds:
 
 Fractional seconds support for Time and DateTime fields