entc/gen: add support for WithNamed<E> feature-flag (#2792)

* entc/gen: struct fields and methods for NamedEdge api

* entc/gen: generate WithNamedEdge methods for named-edges

* entc/gen: implement eager-loading for named-edges

* entc/gen: simplify eager-loading template

* entc/gen: drop support for unqiue edges in named-based loading

* all: codegen

* doc/website: named-edges feature-flag

* Update doc/md/eager-load.mdx

* Update doc/md/eager-load.mdx

entc/gen/template/dialect/sql/feature/migratediff.tmpl

@@ -0,0 +1,40 @@
+{{/*
+Copyright 2019-present Facebook Inc. All rights reserved.
+This source code is licensed under the Apache 2.0 license found
+in the LICENSE file in the root directory of this source tree.
+*/}}
+
+{{/* gotype: entgo.io/ent/entc/gen.Graph */}}
+
+{{ define "migrate/diff" }}
+// Diff compares the state read from a database connection or migration directory with
+// the state defined by the Ent schema. Changes will be written to new migration files.
+func Diff(ctx context.Context, url string, opts ...schema.MigrateOption) error {
+    return NamedDiff(ctx, url, "changes", opts...)
+}
+
+// NamedDiff compares the state read from a database connection or migration directory with
+// the state defined by the Ent schema. Changes will be written to new named migration files.
+func NamedDiff(ctx context.Context, url, name string, opts ...schema.MigrateOption) error {
+    return schema.Diff(ctx, url, name, Tables, opts...)
+}
+// Diff creates a migration file containing the statements to resolve the diff
+// between the Ent schema and the connected database.
+func (s *Schema) Diff(ctx context.Context, opts ...schema.MigrateOption) error {
+    migrate, err := schema.NewMigrate(s.drv, opts...)
+    if err != nil {
+        return fmt.Errorf("ent/migrate: %w", err)
+    }
+    return migrate.Diff(ctx, Tables...)
+}
+
+// NamedDiff creates a named migration file containing the statements to resolve the diff
+// between the Ent schema and the connected database.
+func (s *Schema) NamedDiff(ctx context.Context, name string, opts ...schema.MigrateOption) error {
+    migrate, err := schema.NewMigrate(s.drv, opts...)
+    if err != nil {
+        return fmt.Errorf("ent/migrate: %w", err)
+    }
+    return migrate.NamedDiff(ctx, name, Tables...)
+}
+{{ end }}