qe: fix implicit many-to-many relations in multiSchema context

We did not add any schema prefix there.

We now add the correct one (the schema of the first model).

closes prisma/prisma#16761

migration-engine/migration-engine-tests/tests/single_migration_tests/postgres/multi_schema_implicit_many_to_many_join_table_is_in_first_model_schema.prisma

@@ -0,0 +1,64 @@
+// tags=postgres
+// exclude=cockroachdb
+
+datasource testds {
+    provider = "postgresql"
+    url = env("TEST_DATABASE_URL")
+    schemas = ["veggies", "roots"]
+}
+
+generator js {
+    provider = "prisma-client-js"
+    previewFeatures = ["multiSchema"]
+}
+
+model zoodles {
+  id Int @id
+  shiratakies shirataki[]
+  @@schema("veggies")
+}
+
+model shirataki {
+  id Int @id
+  zoodles zoodles[]
+  @@schema("roots")
+}
+
+// Expected Migration:
+// -- CreateSchema
+// CREATE SCHEMA IF NOT EXISTS "roots";
+// 
+// -- CreateSchema
+// CREATE SCHEMA IF NOT EXISTS "veggies";
+// 
+// -- CreateTable
+// CREATE TABLE "veggies"."zoodles" (
+//     "id" INTEGER NOT NULL,
+// 
+//     CONSTRAINT "zoodles_pkey" PRIMARY KEY ("id")
+// );
+// 
+// -- CreateTable
+// CREATE TABLE "roots"."shirataki" (
+//     "id" INTEGER NOT NULL,
+// 
+//     CONSTRAINT "shirataki_pkey" PRIMARY KEY ("id")
+// );
+// 
+// -- CreateTable
+// CREATE TABLE "roots"."_shiratakiTozoodles" (
+//     "A" INTEGER NOT NULL,
+//     "B" INTEGER NOT NULL
+// );
+// 
+// -- CreateIndex
+// CREATE UNIQUE INDEX "_shiratakiTozoodles_AB_unique" ON "roots"."_shiratakiTozoodles"("A", "B");
+// 
+// -- CreateIndex
+// CREATE INDEX "_shiratakiTozoodles_B_index" ON "roots"."_shiratakiTozoodles"("B");
+// 
+// -- AddForeignKey
+// ALTER TABLE "roots"."_shiratakiTozoodles" ADD CONSTRAINT "_shiratakiTozoodles_A_fkey" FOREIGN KEY ("A") REFERENCES "roots"."shirataki"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+// 
+// -- AddForeignKey
+// ALTER TABLE "roots"."_shiratakiTozoodles" ADD CONSTRAINT "_shiratakiTozoodles_B_fkey" FOREIGN KEY ("B") REFERENCES "veggies"."zoodles"("id") ON DELETE CASCADE ON UPDATE CASCADE;

query-engine/connector-test-kit-rs/query-engine-tests/tests/new/multi_schema.rs

@@ -25,6 +25,26 @@ mod multi_schema {
         schema.to_owned()
     }
 
+    pub fn multi_schema_implicit_m2m() -> String {
+        let schema = indoc! {
+            r#"
+                model Loop {
+                    id Int @id
+                    fruits Fruit[]
+                    @@schema("shapes")
+                }
+
+                model Fruit {
+                    id Int @id
+                    loops Loop[]
+                    @@schema("objects")
+                }
+            "#
+        };
+
+        schema.to_owned()
+    }
+
     #[connector_test(schema(multi_schema_simple), db_schemas("schema1", "schema2"))]
     async fn crud_simple(runner: Runner) -> TestResult<()> {
         // CREATE
@@ -516,4 +536,18 @@ mod multi_schema {
 
         Ok(())
     }
+
+    #[connector_test(schema(multi_schema_implicit_m2m), db_schemas("shapes", "objects"))]
+    async fn implicit_m2m_simple(runner: Runner) -> TestResult<()> {
+        let result = runner
+            .query(r#"mutation { createOneFruit(data: { id: 1, loops: { create: [{ id: 11 }, { id: 12 }] }}) { id loops { id } } }"#)
+            .await?;
+        result.assert_success();
+        let result = result.to_string();
+        assert_eq!(
+            result,
+            "{\"data\":{\"createOneFruit\":{\"id\":1,\"loops\":[{\"id\":11},{\"id\":12}]}}}"
+        );
+        Ok(())
+    }
 }

query-engine/connectors/sql-query-connector/src/model_extensions/relation.rs

@@ -67,8 +67,11 @@ impl AsTable for Relation {
             // table, so MSSQL can convert the `INSERT .. ON CONFLICT IGNORE` into
             // a `MERGE` statement.
             RelationLinkManifestation::RelationTable(ref m) => {
-                let db = self.model_a().internal_data_model().db_name.clone();
-                let table: Table = (db, m.table.clone()).into();
+                let model_a = self.model_a();
+                let prefix = model_a
+                    .schema_name()
+                    .unwrap_or_else(|| model_a.internal_data_model().db_name.clone());
+                let table: Table = (prefix, m.table.clone()).into();
 
                 table.add_unique_index(vec![Column::from("A"), Column::from("B")])
             }