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Starting from an existing database

We'll redo the same tutorial, but this time starting from an existing database to show how you can do the same with your own database.

Setting up a new database​

Let's use ddl.sql and seed.sql from the financial-advisor-mcp directory to create a new database (which is the same as the one we used in the existing Exograph model tutorial).

createdb financial-advisor-mcp
psql -U postgres -d financial-advisor-mcp -f ddl.sql
psql -U postgres -d financial-advisor-mcp -f seed.sql

Creating a new Exograph model from the database​

Use the exo new command to create a new Exograph model from the database:

DATABASE_URL=postgresql://localhost:5432/financial-advisor-mcp exo new financial-advisor-mcp-scratch --query-access true

This creates a new Exograph model in the financial-advisor-mcp-scratch directory. The --query-access true flag sets up access control rules so that anyone can query any data (but no one can mutate it).

Open the financial-advisor-mcp-scratch/index.exo file. You should see the Exograph model for the financial advisor domain (and all access control annotations set to @access(query=true, mutate=false)).

Starting the MCP server​

Before starting the MCP server, create a .env.dev.local file in the financial-advisor-mcp-scratch directory:

echo "DATABASE_URL=postgresql://localhost:5432/financial-advisor-mcp" > financial-advisor-mcp-scratch/.env.dev.local

Use the exo dev command to start the MCP server:

exo dev

Update the Claude Desktop configuration file to use the MCP server:

{
  "mcpServers": {
    "finance-advisory-local": {
      "command": "<your home directory>/.exograph/bin/exo-mcp-bridge",
      "args": [
        "--endpoint",
        "http://localhost:9876/mcp"
      ]
    }
  }
}

Since the model in the current version doesn't require authentication, we don't need to pass the Authorization header.

Now start Claude Desktop, open a new chat window, and try the same prompts as before.

Applying access control rules​

First, add a context element to the index.exo file to extract the current user's ID and role:

context AuthContext {
  @jwt("sub") id: Uuid
  @jwt role: String
}

Next, use the AuthContext in the index.exo file to tighten the access control rules. We want to restrict the Account type so it can be viewed only by:

  • The associated customer (self.customer.id == AuthContext.id)
  • The customer's financial advisor (self.customer.financialAdvisor.id == AuthContext.id)
  • An admin (AuthContext.role == "admin")

So the access control rule would be:

AuthContext.id == self.customer.id || 
AuthContext.id == self.customer.financialAdvisor.id ||
AuthContext.role == "admin"

Set this access control rule on the Account type as follows:

@access(query=AuthContext.id == self.customer.id || 
              AuthContext.id == self.customer.financialAdvisor.id ||
              AuthContext.role == "admin")
type Account {
  ...
}

Since Exograph's default access control is false, we don't need to specify @access(mutate=false) on the Account type.

Similarly, for the Customer type, we want to ensure it can be viewed only by:

  • The customer themselves (self.id == AuthContext.id)
  • The associated financial advisor (self.financialAdvisor.id == AuthContext.id)
  • An admin (AuthContext.role == "admin")

So the access control rule would be:

AuthContext.id == self.id || AuthContext.id == self.financialAdvisor.id || AuthContext.role == "admin"

Set this access control rule on the Customer type as follows:

@access(query=AuthContext.id == self.id || 
              AuthContext.id == self.financialAdvisor.id || 
              AuthContext.role == "admin")
type Customer {
  ...
}

You can now update the rest of the types in a similar manner. Study access control rules for more details or take a peek at the code from the existing Exograph model tutorial.

Using the MCP server​

Using the MCP server is the same as in the existing Exograph model tutorial. Make sure to pass the Authorization header, since with access control in place, you'll get errors without it (except for types that have access control set to @access(query=true, mutate=false)).