Populate DatabaseSchema by reading it from database

[Aniruddh25]

Summary

Today, FileMetadataProvider reads the runtime config file sql-config.json to get and populate DatabaseSchema object. This change implements a SqlMetadataProvider to read the same information from the database instead.

How

• Use DbConnection.GetSchema("Tables") to get all the base tables. Ref : https://docs.microsoft.com/en-us/dotnet/framework/data/adonet/sql-server-schema-collections
• Use DbDataAdapter's FillSchema() method to obtain the primary key, IsNullable and IsAutoGenerated information for each of the columns in each table. Ref: https://docs.microsoft.com/en-us/dotnet/framework/data/adonet/adding-existing-constraints-to-a-dataset
• The Default column does not return accurate information. So, we have to use DbConnection.GetSchema("Columns") to determine if the column has default value or not.
• The difference between MsSql, PgSql and MySql are only the DbConnection, DbDataAdapter and DbCommand. We use generic arguments to denote the difference.

Tests

Added unit tests to verify the DatabaseSchema derived from the database matches what we expect from the runtime config file provider.

Future work:

• To obtain foreign key information as well.
• To use this MetadataProvider instead of the filemetadataprovider for the DatabaseSchema
• To get ColumnType information
• To separate this out into a library and reuse it in the GraphQL Schema Builder project

[junsu0ms]

Both GetSchema and FillSchema are interface of ADO.NET. So it should work with MySQL/PGSQL?

[Aniruddh25]

Both GetSchema and FillSchema are interface of ADO.NET. So it should work with MySQL/PGSQL?

Thats what Im hoping for. Need to test it.

[seantleonard]

Thanks for adding this! Looks good, just some clarifying questions and comments.

[seantleonard]

Does this cause issues if requests come in between clearing the schema and re-populating it?

[Aniruddh25]

It might, I have added this just envisioning we might need to support dynamic changes as per outcome of https://github.com/Azure/project-hawaii/issues/76
I think it might be reasonable to expect issues for a short period until we re-populate it if we decide to support the dynamic changes.

All the changes requested have been addressed. Merging it to unblock future development.