[Bug]: Postgres dbName is ignored

[noahfraiture]

Testcontainers version

0.40.0

Using the latest Testcontainers version?

Yes

Host OS

MacOS

Host arch

ARM

Go version

1.25

Docker version

Client:
 Version:           29.1.2
 API version:       1.52
 Go version:        go1.25.5
 Git commit:        890dcca
 Built:             Tue Dec  2 21:54:24 2025
 OS/Arch:           darwin/arm64
 Context:           desktop-linux

Server: Docker Desktop 4.54.0 (212467)
 Engine:
  Version:          29.1.2
  API version:      1.52 (minimum version 1.44)
  Go version:       go1.25.5
  Git commit:       de45c2a
  Built:            Tue Dec  2 21:55:52 2025
  OS/Arch:          linux/arm64
  Experimental:     false
 containerd:
  Version:          v2.2.0
  GitCommit:        1c4457e00facac03ce1d75f7b6777a7a851e5c41
 runc:
  Version:          1.3.4
  GitCommit:        v1.3.4-0-gd6d73eb8
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

Docker info

Client:
 Version:    29.1.2
 Context:    desktop-linux
 Debug Mode: false
 Plugins:
  ai: Docker AI Agent - Ask Gordon (Docker Inc.)
    Version:  v1.17.0
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-ai
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.30.1-desktop.1
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.40.3-desktop.1
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-compose
  debug: Get a shell into any image or container (Docker Inc.)
    Version:  0.0.45
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-debug
  desktop: Docker Desktop commands (Docker Inc.)
    Version:  v0.2.0
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-desktop
  extension: Manages Docker extensions (Docker Inc.)
    Version:  v0.2.31
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-extension
  init: Creates Docker-related starter files for your project (Docker Inc.)
    Version:  v1.4.0
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-init
  mcp: Docker MCP Plugin (Docker Inc.)
    Version:  v0.28.0
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-mcp
  model: Docker Model Runner (Docker Inc.)
    Version:  v1.0.4
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-model
  offload: Docker Offload (Docker Inc.)
    Version:  v0.5.29
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-offload
  pass: Docker Pass Secrets Manager Plugin (beta) (Docker Inc.)
    Version:  v0.0.15
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-pass
  sandbox: Docker Sandbox (Docker Inc.)
    Version:  v0.6.0
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-sandbox
  sbom: View the packaged-based Software Bill Of Materials (SBOM) for an image (Anchore Inc.)
    Version:  0.6.0
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-sbom
  scout: Docker Scout (Docker Inc.)
    Version:  v1.18.3
    Path:     /Users/noah.fraiture/.docker/cli-plugins/docker-scout

Server:
 Containers: 4
  Running: 1
  Paused: 0
  Stopped: 3
 Images: 61
 Server Version: 29.1.2
 Storage Driver: overlayfs
  driver-type: io.containerd.snapshotter.v1
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Discovered Devices:
  cdi: docker.com/gpu=webgpu
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 1c4457e00facac03ce1d75f7b6777a7a851e5c41
 runc version: v1.3.4-0-gd6d73eb8
 init version: de40ad0
 Security Options:
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.12.54-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: aarch64
 CPUs: 14
 Total Memory: 7.653GiB
 Name: docker-desktop
 ID: aa92124a-5e45-4832-ac5c-d2b0e50e6f4e
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 HTTP Proxy: http.docker.internal:3128
 HTTPS Proxy: http.docker.internal:3128
 No Proxy: hubproxy.docker.internal
 Labels:
  com.docker.desktop.address=unix:///Users/noah.fraiture/Library/Containers/com.docker.docker/Data/docker-cli.sock
 Experimental: false
 Insecure Registries:
  hubproxy.docker.internal:5555
  ::1/128
  127.0.0.0/8
 Live Restore Enabled: false
 Firewall Backend: iptables

What happened?

postgres.WithDatabase has no effect

It seems that postgres.WithDatabase is no longer working as expected.

Reproduction

containersAttrs = append(containersAttrs, postgres.WithDatabase(dbName))

container, err := postgres.Run(
	t.Context(),
	"postgres:17-alpine",
	containersAttrs...,
)
if err != nil {
	t.Fatal(err)
}

postgresDSNWithDB, err := container.ConnectionString(
	t.Context(),
	"sslmode=disable",
)
if err != nil {
	t.Fatal(err)
}

fmt.Println(postgresDSNWithDB)
// postgres://<default>:password@localhost:55503/postgres?sslmode=disable

Even though WithDatabase(dbName) is provided, the connection string still points to the default postgres database instead of dbName.

Investigation

When inspecting the container, I noticed the following logic:

for _, env := range inspect.Config.Env {
	// POSTGRES_DB=postgres
	if v, ok := strings.CutPrefix(env, "POSTGRES_DB="); ok {
		c.dbName, foundDB = v, true
	}
}

However, WithDatabase is defined as:

func WithDatabase(dbName string) testcontainers.ContainerCustomizer {
	return testcontainers.WithEnv(map[string]string{
		"POSTGRES_DB": dbName,
	})
}

The customizer is successfully applied, but the resulting container still behaves as if POSTGRES_DB were set to the default value.

Regression

This issue did not occur with testcontainers-go version 0.38.0 and appears to be a regression in newer versions.

Relevant log output

Additional information

No response

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🔗 Similar Issues

Related Issues

• [Bug]: Multiple errors by PostgreSQL snapshot restore #3263
• [Bug]: cockroachdb connection string #3104
• [Bug]: Error on postgres restore #3233
• [Bug]: panic when connecting to colima 0.9.1 #3460

🔗 Related PRs

#3414 - chore(k6|localstack|kafka|mariadb): use Run function [merged]
#3425 - chore(postgres): use Run function [merged]

👤 Suggested Assignees

• mdelapenya
• ascheman
• rmanibus
• aBMania
• jmhodges

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[mdelapenya]

Hi @noahfraiture is this bug reproduced in your code? I'm, currently debugging the TestPostgres test from the library, where we use the WithDatabase, and confirm the database is set through the env variable, see attachment:

Can you elaborate a bit more? Thanks!

[noahfraiture]

Hello, yes of course.
We have a lot of tests and the logic is the following for each tests:

• Spawn a container with reuse name
  • If the container exist, reuse
  • If the container doesn't exist, spawn one
• Generate a database name, reopen a container with same same reuse name and now WithDatabase

Previously because the package was reading the database from the env variable, it was working fine. But a recent update changed that. We have a workaround which is simply to generate the DSN ourselves with the database name we want.
Is this change of behavior a choice ?
This seems to be the change https://github.com/testcontainers/testcontainers-go/pull/3425/files