claim.yaml

    ...
    env-vars:
    - name: PG_USERNAME
      valueFrom:
        secretKeyRef:
          key: username
    - name: PG_USERNAME
      valueFrom:
        secretKeyRef:
          key: username
    - name: PG_PASSWORD
      valueFrom:
        secretKeyRef:
          key: password
    - name: DATABASE_HOST
      valueFrom:
        secretKeyRef:
          key: endpoint
    - name: DATABASE_NAME
      value: my_db
    - name: POSTGRES_URL
      value: ecto://$(PG_USERNAME):$(PG_PASSWORD)@$(DATABASE_HOST):5432/$(DATABASE_NAME)
...

composition.yaml

...
- name: application-deployment
    base:
      apiVersion: kubernetes.crossplane.io/v1alpha1
      kind: Object
      spec:
        forProvider:
          manifest:
            apiVersion: apps/v1
            kind: Deployment
            spec:
              template:
                spec:
                  containers:
                    - ports:
                        - containerPort: 4000                      
                  imagePullSecrets:
                    - name: regcred
    patches:
      - fromFieldPath: spec.parameters.env-vars
        toFieldPath: spec.forProvider.manifest.spec.template.spec.containers[0].env
        policy:
          mergeOptions:
            appendSlice: true

crd.yaml

...
env-vars:
  type: array
  items:
    type: object
    properties:
      name:
        type: string
      value:
        type: string
      valueFrom:
        type: object
        properties:
          configMapKeyRef:
            type: object
            properties:
              key:
                type: string
              name:
                type: string
          secretKeyRef:
            type: object
            properties:
              key:
                type: string
              name:
                type: string