Added template function where

Author: Ted Chen

README.md

@@ -132,6 +132,7 @@ Within those templates, the object emitted by docker-gen will have [this structu
 * *`split $string $sep`*: Splits `$string` into a slice of substrings delimited by `$sep`. Alias for [`strings.Split`](http://golang.org/pkg/strings/#Split)
 * *`trimPrefix $prefix $string`*: If `$prefix` is a prefix of `$string`, return `$string` with `$prefix` trimmed from the beginning. Otherwise, return `$string` unchanged.
 * *`trimSuffix $suffix $string`*: If `$suffix` is a suffix of `$string`, return `$string` with `$suffix` trimmed from the end. Otherwise, return `$string` unchanged.
+* *`where $containers $fieldPath $value`*: Filters an array of `RuntimeContainer` instances based on the values of a field path expression `$fieldPath`. A field path expression is a dot-delimited list of map keys or struct member names specifying the path from container to a nested value, which must be a string. Returns an array of containers having that value.
 
 ===
 

template.go

@@ -65,6 +65,18 @@ func groupByKeys(entries []*RuntimeContainer, key string) []string {
 	return ret
 }
 
+// selects entries based on key
+func where(entries []*RuntimeContainer, key string, cmp string) []*RuntimeContainer {
+	selection := []*RuntimeContainer{}
+	for _, v := range entries {
+		value := deepGet(*v, key)
+		if value == cmp {
+			selection = append(selection, v)
+		}
+	}
+	return selection
+}
+
 // hasPrefix returns whether a given string is a prefix of another string
 func hasPrefix(prefix, s string) bool {
 	return strings.HasPrefix(s, prefix)
@@ -240,6 +252,7 @@ func generateFile(config Config, containers Context) bool {
 		"split":        strings.Split,
 		"trimPrefix":   trimPrefix,
 		"trimSuffix":   trimSuffix,
+		"where":		where,
 	}).ParseFiles(templatePath)
 	if err != nil {
 		log.Fatalf("unable to parse template: %s", err)

template_test.go

@@ -176,6 +176,51 @@ func TestGroupByMulti(t *testing.T) {
 	}
 }
 
+func TestWhere(t *testing.T) {
+	containers := []*RuntimeContainer{
+		&RuntimeContainer{
+			Env: map[string]string{
+				"VIRTUAL_HOST": "demo1.localhost",
+			},
+			ID: "1",
+		},
+		&RuntimeContainer{
+			Env: map[string]string{
+				"VIRTUAL_HOST": "demo2.localhost",
+			},
+			ID: "2",
+		},
+		&RuntimeContainer{
+			Env: map[string]string{
+				"VIRTUAL_HOST": "demo3.localhost",
+			},
+			ID: "3",
+		},
+		&RuntimeContainer{
+			Env: map[string]string{
+				"VIRTUAL_HOST": "demo2.localhost",
+			},
+			ID: "4",
+		},
+	}
+
+	if len(where(containers, "Env.VIRTUAL_HOST", "demo1.localhost")) != 1 {
+		t.Fatalf("expected 1 match")
+	}
+
+	if len(where(containers, "Env.VIRTUAL_HOST", "demo2.localhost")) != 2 {
+		t.Fatalf("expected 2 matches")
+	}
+
+	if len(where(containers, "Env.VIRTUAL_HOST", "demo3.localhost")) != 1 {
+		t.Fatalf("expected 1 match")
+	}
+
+	if len(where(containers, "Env.NOEXIST", "demo3.localhost")) != 0 {
+		t.Fatalf("expected 0 match")
+	}
+}
+
 func TestHasPrefix(t *testing.T) {
 	const prefix = "tcp://"
 	const str = "tcp://127.0.0.1:2375"