add offset step to query builder (#1049)

Summary:
Pull Request resolved: https://github.com/facebookexternal/fbc/pull/1049

Useful for real paging

Reviewed By: noamsch

Differential Revision: D16003607

fbshipit-source-id: 6a85d0e3d71a2582bc3cd8f1d66748dda4f2a10e

entc/gen/template/builder/query.tmpl

@@ -10,10 +10,11 @@
 // {{ $builder }} is the builder for querying {{ pascal $.Name }} entities.
 type {{ $builder }} struct {
 	config
-	limit *int
-	order  []Order
-	unique []string
-	predicates []ent.Predicate
+	limit		*int
+	offset		*int
+	order		[]Order
+	unique		[]string
+	predicates 	[]ent.Predicate
 	// intermediate queries.
 	sql     *sql.Selector
 	gremlin *dsl.Traversal
@@ -31,6 +32,12 @@ func ({{ $receiver }} *{{ $builder }}) Limit(limit int) *{{ $builder }} {
 	return {{ $receiver }}
 }
 
+// Offset adds an offset step to the query.
+func ({{ $receiver }} *{{ $builder }}) Offset(offset int) *{{ $builder }} {
+	{{ $receiver }}.offset = &offset
+	return {{ $receiver }}
+}
+
 // Order adds an order step to the query.
 func ({{ $receiver }} *{{ $builder }}) Order(o ...Order) *{{ $builder }} {
 	{{ $receiver }}.order = append({{ $receiver }}.order, o...)

entc/gen/template/dialect/gremlin/query.tmpl

@@ -65,7 +65,12 @@ func ({{ $receiver }} *{{ $builder }}) gremlinQuery() *dsl.Traversal {
 			p.Gremlin(v)
 		}
 	}
-	if limit := {{ $receiver }}.limit; limit != nil {
+	switch limit, offset := {{ $receiver }}.limit, {{ $receiver }}.offset; {
+	case limit != nil && offset != nil:
+		v.Range(*offset, *offset + *limit)
+	case offset != nil:
+		v.Range(*offset, math.MaxInt64)
+	case limit != nil:
 		v.Limit(*limit)
 	}
 	if unique := {{ $receiver }}.unique; len(unique) == 0 {

entc/gen/template/dialect/sql/query.tmpl

@@ -80,6 +80,11 @@ func ({{ $receiver }} *{{ $builder }}) sqlQuery() *sql.Selector {
 	for _, p := range {{ $receiver }}.order {
 		p.SQL(selector)
 	}
+	if offset := {{ $receiver }}.offset; offset != nil {
+		// limit is mandatory for offset clause. We start
+		// with default value, and override it below if needed.
+		selector.Offset(*offset).Limit(math.MaxInt64)
+	}
 	if limit := {{ $receiver }}.limit; limit != nil {
 		selector.Limit(*limit)
 	}

entc/gen/template/import.tmpl

@@ -3,6 +3,7 @@ import (
 	"fmt"
 	"context"
 	"errors"
+	"math"
 	"strconv"
 	"strings"
 	"time"