Address concerns around count typings

types/index.d.ts

@@ -11,6 +11,7 @@
 import events = require('events');
 import stream = require('stream');
 import Bluebird = require('bluebird');
+import ResultTypes = require('./result');
 
 // # Generic type-level utilities
 
@@ -86,7 +87,7 @@ type MappedAliasType<TBase, TAliasMapping> = {} & {
 // but the keys being selected or additional properties being augmented are not
 // all known at once and we would want to effectively build up a partial/intersection
 // over multiple steps.
-interface DeferredKeySelection<
+type DeferredKeySelection<
   // The base of selection. In intermediate stages this may be unknown.
   // If it remains unknown at the point of resolution, the selection will fall back to any
   TBase,
@@ -106,7 +107,7 @@ interface DeferredKeySelection<
   TIntersectProps extends {} = {},
   // Extra props which will be unioned with the result
   TUnionProps = never
-> {
+> = {
   // These properties are not actually used, but exist simply because
   // typescript doesn't end up happy when type parameters are unused
   _base: TBase;
@@ -116,7 +117,7 @@ interface DeferredKeySelection<
   _single: TSingle;
   _intersectProps: TIntersectProps;
   _unionProps: TUnionProps;
-}
+};
 
 // An companion namespace for DeferredKeySelection which provides type operators
 // to build up participants of intersection/partial over multiple invocations
@@ -273,8 +274,7 @@ type AggregationQueryResult<TResult, TIntersectProps2> = ArrayIfAlready<
 // deferring an index access operation (TBase[TKey]) over a potentially
 // unknown initial type of TBase and potentially never initial type of TKey
 
-interface DeferredIndex<TBase, TKey extends string>
-  extends DeferredKeySelection<TBase, TKey, false, {}, true> {}
+type DeferredIndex<TBase, TKey extends string> = DeferredKeySelection<TBase, TKey, false, {}, true>;
 
 declare namespace DeferredIndex {
   type Augment<
@@ -297,9 +297,7 @@ type ResolveResult<S> = DeferredKeySelection.Resolve<S>;
 type Callback = Function;
 type Client = Function;
 
-interface Dict<T = any> {
-  [k: string]: T;
-}
+type Dict<T = any> = { [k: string]: T; };
 
 type SafePick<T, K extends keyof T> = T extends {} ? Pick<T, K> : any;
 
@@ -382,6 +380,11 @@ declare namespace Knex {
 
   type TableDescriptor = string | Knex.Raw | Knex.QueryBuilder;
 
+  type Lookup<TRegistry extends {}, TKey extends string, TDefault = never> =
+    TKey extends keyof TRegistry ?
+      TRegistry[TKey] :
+      TDefault;
+
   //
   // QueryInterface
   //
@@ -498,8 +501,8 @@ declare namespace Knex {
     limit(limit: number): QueryBuilder<TRecord, TResult>;
 
     // Aggregation
-    count: AssymetricAggregation<TRecord, TResult, number | string>;
-    countDistinct: AssymetricAggregation<TRecord, TResult, number | string>;
+    count: AssymetricAggregation<TRecord, TResult, Lookup<ResultTypes.Registry, "Count", number | string>>;
+    countDistinct: AssymetricAggregation<TRecord, TResult, Lookup<ResultTypes.Registry, "Count", number | string>>;
     min: TypePreservingAggregation<TRecord, TResult>;
     max: TypePreservingAggregation<TRecord, TResult>;
     sum: TypePreservingAggregation<TRecord, TResult>;

types/result.d.ts

@@ -0,0 +1,27 @@
+// This empty interface serves as a placeholder which userland code can augment to
+// override result types.
+//
+// Currently only available result type which is overridable is Count, which defaults to
+// number | string;
+//
+// Following example in userland code will alter this to be just number:
+//
+// declare module "knex/types/result" {
+//     interface Registry {
+//         Count: number;
+//     }
+// }
+//
+// Prior discussion: https://github.com/tgriesser/knex/issues/3247
+export interface Registry {
+    // We can't actually have default types here
+    // because typescript's augmentation will not permit
+    // overriding the type of a property already present.
+    //
+    // But the effective defaults are documented below:
+    //
+    // Count: number | string;
+    //
+    // Refer to Knex.Lookup type operator to see how the defaults
+    // are actually specified.
+}

types/tslint.json

@@ -9,6 +9,7 @@
     "unified-signatures": false,
     "no-unnecessary-qualifier": false,
     "strict-export-declare-modifiers": false,
-    "only-arrow-functions": false
+    "only-arrow-functions": false,
+    "interface-over-type-literal": false
   }
 }