persistent-mysql: truncate foreign key constraint names to MySQL's 64 char limit

persistent-mysql/Database/Persist/MySQL.hs

@@ -1766,6 +1766,35 @@ putManySql' (filter isFieldNotGenerated -> fields) ent n = q
             , Util.commaSeparated updates
             ]
 
+-- | Generate the default foreign key constraint name for a given source table and
+-- source column name, truncated to fit MySQL's 64 character identifier limit.
+--
+-- MySQL, unlike Postgres, does not silently truncate identifiers that are too long;
+-- it raises an error instead (\"Identifier name ... is too long\"). Without this,
+-- persistent would suggest foreign key constraint names that MySQL simply refuses to
+-- create. This mirrors the truncation approach @persistent-postgresql@ already uses
+-- for the same underlying problem, just with MySQL's 64 character limit instead of
+-- Postgres' 63.
+refName :: EntityNameDB -> FieldNameDB -> ConstraintNameDB
+refName (EntityNameDB table) (FieldNameDB column) =
+    let
+        overhead = T.length $ T.concat ["_", "_fkey"]
+        (fromTable, fromColumn) = shortenNames overhead (T.length table, T.length column)
+     in
+        ConstraintNameDB $
+            T.concat [T.take fromTable table, "_", T.take fromColumn column, "_fkey"]
+  where
+    maximumIdentifierLength :: Int
+    maximumIdentifierLength = 64
+
+    shortenNames :: Int -> (Int, Int) -> (Int, Int)
+    shortenNames overhead (x, y)
+        | x + y + overhead <= maximumIdentifierLength = (x, y)
+        | x > y = shortenNames overhead (x - 1, y)
+        | otherwise = shortenNames overhead (x, y - 1)
+
 mysqlMkColumns
     :: [EntityDef] -> EntityDef -> ([Column], [UniqueDef], [ForeignDef])
-mysqlMkColumns allDefs t = mkColumns allDefs t emptyBackendSpecificOverrides
+mysqlMkColumns allDefs t =
+    mkColumns allDefs t $
+        setBackendSpecificForeignKeyName refName emptyBackendSpecificOverrides

persistent-mysql/persistent-mysql.cabal

@@ -1,5 +1,5 @@
 name:               persistent-mysql
-version:            2.13.1.6
+version:            2.13.1.7
 license:            MIT
 license-file:       LICENSE
 author:             Felipe Lessa <felipe.lessa@gmail.com>, Michael Snoyman

persistent-mysql/test/main.hs

@@ -158,8 +158,8 @@ main = do
             , CustomPrimaryKeyReferenceTest.migration
             , MigrationColumnLengthTest.migration
             , TransactionLevelTest.migration
-            , -- , LongIdentifierTest.migration
-              ForeignKey.compositeMigrate
+            , LongIdentifierTest.migration
+            , ForeignKey.compositeMigrate
             ]
         PersistentTest.cleanDB
         ForeignKey.cleanDB
@@ -237,10 +237,7 @@ main = do
         MigrationIdempotencyTest.specsWith db
         MigrationTest.specsWith db
         CustomConstraintTest.specs db
-        -- TODO: implement automatic truncation for too long foreign keys, so we can run this test.
-        xdescribe
-            "The migration for this test currently fails because of MySQL's 64 character limit for identifiers. See https://github.com/yesodweb/persistent/issues/1000 for details"
-            $ LongIdentifierTest.specsWith db
+        LongIdentifierTest.specsWith db
         GeneratedColumnTestSQL.specsWith db
         JSONTest.specs
 

persistent-test/src/LongIdentifierTest.hs

@@ -19,7 +19,7 @@ import Init
 
 -- This test creates very long identifier names. The generated foreign key is over the length limit for Postgres and MySQL
 -- persistent-postgresql handles this by truncating foreign key names using the same algorithm that Postgres itself does (see 'refName' in Postgresql.hs)
--- MySQL currently doesn't run this test, and needs truncation logic for it to pass.
+-- persistent-mysql does the same, using MySQL's 64 character limit (see 'refName' in MySQL.hs)
 share
     [mkPersist sqlSettings, mkMigrate "migration"]
     [persistLowerCase|