Having some issues with docker-compose - postgresql

Question

Im pretty new to flask , postgres and docker. I am trying to dockerize our application so that a new dev does not have to worry about installing python depenedencies to start developing. I currently have two containers, one for the flask app and one for the postgres db.
The issue I am having is that when I put the two in a compose they cannot connect to each other. I have defined my own network to ensure that they are running on the same one but that still does not seem to solve the problem. When I run the flask app outside of a container and try to connect to the containerized postgres db I am having no trouble connecting and it works fine.
Any help would be much appreciated
Thanks in advance
Dockerfile for the flask app
FROM python:3.7-alpine3.7
LABEL maintainer="uwblueprint"
LABEL org.label-schema.schema-version="1.0"
LABEL org.label-schema.name="elevate-api"
LABEL org.label-schema.vcs-url="https://gitlab.com/uwblueprint/elevate-api/"
LABEL org.label-schema.vendor="UW Blueprint"
## Copy source and environment-config files.
COPY app/ ./app/
WORKDIR /app
COPY Pipfile Pipfile.lock ./
## Install external dependencies.
RUN apk add --no-cache libpq
## Install application dependencies.
RUN apk add --no-cache --virtual build-deps \
gcc musl-dev postgresql-dev libffi-dev && \
pip3 install --upgrade pip pipenv gunicorn && \
pipenv --python 3.7 && \
pipenv install --system --deploy && \
apk del build-deps
EXPOSE 5000
CMD ["flask", "run"]
Docker Compose File
version: '3.7'
services:
database:
image: registry.gitlab.com/uwblueprint/elevate-api/postgres:latest
build:
context: ./external/postgres
cache_from:
- registry.gitlab.com/uwblueprint/elevate-api/postgres:latest
- postgres:10.5-alpine
container_name: database
env_file: ./external/postgres/configs/.env
volumes:
- postgres.data:/var/lib/postgresql/data # persist data
ports:
- "5432:5432"
networks:
- "api_net"
api:
build: .
ports:
- "5000:5000"
networks:
- "api_net"
networks:
api_net:
volumes:
postgres.data:
Python SQL Alchemy Code
import os
from flask_sqlalchemy import SQLAlchemy
from app import app
# Make sure that DB_PASS is an environment variable:
##if 'DB_PASS' not in os.environ:
## raise EnvironmentError("Could not find environment variable 'DB_PASS',")
##user = os.environ.get("DB_USER", "robot")
user ="fakeUser"
password = "fakePassword"
##password = os.environ["DB_PASS"]
host = os.environ.get("DB_HOST", "0.0.0.0")
port = os.environ.get("DB_PORT", "5432")
name = os.environ.get("DB_NAME", "elevate")
# Configure flask-sqlalchemy.
app.config["SQLALCHEMY_DATABASE_URI"] = "postgresql://%s:%s#%s:%s/%s" % (
user, password, host, port, name
)
app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = False
# TODO: Find the optimal pool recycle time for PostgreSQL.
app.config["SQLALCHEMY_POOL_RECYCLE"] = 7200 # in milliseconds
# The db object is aware of the Flask application lifecycle, and will do things
# like close the database session when a Flask request ends.
#
# This saves a lot of headaches compared to the plain SQLAlchemy library, where
# I kept running into issues about database-session related objects trying
# to access the session when I had closed it prematurely (due to SQLAlchemy's
# lazy-loading data acess methods).
db = SQLAlchemy(app)

Answer 1

Set DB_HOST to "database" which is the name of the container.

Answer 2

Inside docker-compose you can make connection between services by service names. You can read about this in documentation.
By default Compose sets up a single network for your app. Each container for a service joins the default network and is both reachable by other containers on that network, and discoverable by them at a hostname identical to the container name.
Set host or DB_HOST environ to database. 127.0.0.1 won't work.